Thermal Storage in Pressurized Fluid for Compressed Air Energy Storage Systems

ABSTRACT

A thermal storage subsystem may include at least a first storage reservoir configured to contain a thermal storage liquid at a storage pressure that is greater than atmospheric pressure. A liquid passage may have an inlet connectable to a thermal storage liquid source and configured to convey the thermal storage liquid to the liquid reservoir. A first heat exchanger may be provided in the liquid inlet passage and may be in fluid communication between the first compression stage and the accumulator, whereby thermal energy can be transferred from a compressed gas stream exiting a gas compressor/expander subsystem to the thermal storage liquid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.18/168,078, filed Feb. 13, 2023, which is a continuation of U.S. patentapplication Ser. No. 17/526,508 filed Nov. 15, 2021, now U.S. Pat. No.11,644,150, which is a continuation of U.S. patent application Ser. No.16/492,401 filed Sep. 9, 2019, now U.S. Pat. No. 11,274,792, which is a371 national stage of International Patent Application No.PCT/CA2018/050282, filed Mar. 9, 2018, which claims priority to U.S.Provisional Patent Application Ser. No. 62/469,264, filed Mar. 9, 2017and priority to International Patent Application No. PCT/CA2018/050112,filed Jan. 31, 2018, the entirety of these applications beingincorporated by reference herein.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to compressed gas energystorage, and more particularly to a compressed gas energy storage systemsuch as, for example, one including a hydrostatically compensated,substantially isobaric compressed air energy storage accumulator locatedunderground, the use thereof, as well as a method of storing compressedgas. The present disclosure also relates generally to a system andmethod for providing a system for keeping a heated fluid, such as water,in a liquid state and at a pressure that allows fluid to accept greaterheat to store and release than would be practical under atmosphericconditions.

BACKGROUND

Electricity storage is highly sought after, in view of the costdisparities incurred when consuming electrical energy from a power gridduring peak usage periods, as compared to low usage periods. Theaddition of renewable energy sources, being inherently of adiscontinuous or intermittent supply nature, increases the demand foraffordable electrical energy storage worldwide.

Thus there exists a need for effectively storing the electrical energyproduced at a power grid or a renewable source during a non-peak periodand providing it to the grid upon demand. Furthermore, to the extentthat the infrastructural preparation costs and the environmental impactfrom implementing such infrastructure are minimized, the utility anddesirability of a given solution is enhanced.

Furthermore, as grids transform and operators look to storage inaddition to renewables to provide power and remove traditional forms ofgeneration that also provide grid stability, such as voltage support, astorage method that offers inertia based synchronous storage is highlydesirable.

SUMMARY

This summary is intended to introduce the reader to the more detaileddescription that follows and not to limit or define any claimed or asyet unclaimed invention. One or more inventions may reside in anycombination or sub-combination of the elements or process stepsdisclosed in any part of this document including its claims and figures.

In accordance with one broad aspect of the teaching described herein,which may be used alone or in combination with any other aspects, acompressed gas energy storage system may include an accumulator havingan interior configured to contain compressed gas when in use, whereinthe accumulator has a primary opening, an upper wall, a lower wall andthe accumulator interior containing a compressed gas being at leastpartially bounded by the upper wall and lower wall. The compressed gasenergy storage system may further include a gas compressor/expandersubsystem spaced apart from the accumulator and comprising at least afirst compression stage having a gas inlet and a gas outlet in fluidcommunication with the accumulator interior via a gas conduit forconveying compressed gas to the accumulator when in a charging mode andfrom the accumulator when in a discharging mode. The compressed gasenergy storage system may further include a thermal storage subsystem,comprising: (i) at least a first storage reservoir configured to containa thermal storage liquid at a storage pressure that is greater thanatmospheric pressure, wherein the first storage reservoir comprises apressurized layer of cover gas above the thermal storage liquid and thelayer of cover gas is formed by a boiling of a portion of the thermalstorage liquid within the first storage reservoir whereby the layer ofcover gas is pressurized to the storage pressure; (ii) a liquid passagehaving an inlet connectable to a thermal storage liquid source andconfigured to convey the thermal storage liquid to the first storagereservoir, and (iii) a first heat exchanger provided in the liquidpassage and in fluid communication between the first compression stageand the accumulator, whereby when the compressed gas energy storagesystem is in the charging mode thermal energy is transferred from acompressed gas stream exiting the gas compressor/expander subsystem tothe thermal storage liquid.

The thermal storage liquid may be heated to a storage temperature priorto entering the first storage reservoir, wherein the storage temperatureis below a boiling temperature of the thermal storage liquid when at thestorage pressure and is above the boiling temperature of the thermalstorage liquid when at atmospheric pressure.

The storage temperature is between about 150 degrees Celsius and about350 degrees Celsius.

The compressed gas within the accumulator may be at an accumulatorpressure, and wherein the storage pressure is less than the accumulatorpressure.

The first storage reservoir may at least partially disposed within theaccumulator.

The thermal storage liquid source may include a source reservoircontaining a quantity of the thermal storage liquid at a sourcetemperature that is less than a storage temperature.

The source reservoir may be external the first storage reservoir.

The gas compressor/expander subsystem may further include a secondcompression stage downstream from the first compression stage and thefirst heat exchanger is fluid communication between the firstcompression stage and the second compression stage, and the thermalstorage subsystem further comprises a second heat exchanger in fluidcommunication between the second compression stage and the accumulator,whereby thermal energy is transferred between the compressed gas streamexiting the second compression stage and the thermal storage liquid.

The gas compressor/expander subsystem may further include a thirdcompression stage downstream from the second compression stage and thesecond heat exchanger is fluid communication between the secondcompression stage and the third compression stage, and the thermalstorage subsystem further comprises a third heat exchanger in fluidcommunication between the third compression stage and the accumulator,whereby thermal energy is transferred between the compressed gas streamexiting the third compression stage and the thermal storage liquid.

The first storage reservoir may include a single chamber having achamber bottom wall, a chamber top wall, a chamber sidewall extendingtherefrom and defining a chamber interior configure to contain thethermal storage liquid.

The single chamber may include a natural underground cavity formed atleast partially of natural rock.

The compressed gas energy storage system may further include a storageliner covering at least a portion of an interior surface of the chamber.

The compressed gas energy storage system may further include anextraction pump in liquid communication with the thermal storage liquidin the first storage reservoir and selectably operable to pump thethermal storage liquid at the storage temperature out of the firststorage reservoir, and wherein when an exit stream of gas is releasedfrom the accumulator, thermal energy is transferred from the thermalstorage liquid pumped out of the first storage reservoir into the exitstream of gas.

The exit stream of gas and the thermal storage liquid pumped out of thefirst storage reservoir may pass through the first heat exchanger.

The first storage reservoir may be disposed entirely under ground.

The compressed gas energy storage system may further include a reservoircooling system for selectably cooling a temperature of the thermalstorage liquid contained in the first storage reservoir, therebyreducing the storage pressure within the first storage reservoir.

When the compressed gas energy storage system is in the dischargingmode, compressed gas may travel from the accumulator to the gascompressor/expander subsystem and at least a portion of the thermalstorage liquid at the storage temperature is withdrawn from the firststorage reservoir and the thermal storage subsystem is operable so thatthermal energy is transferred from at least the portion of the thermalstorage liquid withdrawn from the first storage reservoir to thecompressed gas exiting the accumulator whereby the temperature of thecompressed gas exiting the accumulator is increased before it reachesthe gas compressor/expander subsystem.

When the compressed gas energy storage system is in the discharging modethe compressed gas traveling from the accumulator to the gascompressor/expander subsystem passes through the first heat exchanger toreceive thermal energy from the thermal storage liquid.

The first storage reservoir may be configured as an above-ground vessel.

In accordance with one broad aspect of the teachings described herein,which may be used alone or in combination with any other aspects, acompressed gas energy storage system may include an accumulator havingan interior configured to contain compressed gas when in use. A gascompressor/expander subsystem may be spaced apart from the accumulatorand may include at least a first compression stage having a gas inletand a gas outlet in fluid communication with the accumulator interiorfor conveying compressed gas to the accumulator when in a charging modeand from the accumulator when in a discharging mode. A thermal storagesubsystem may include at least a first storage reservoir disposed atleast partially under ground and configured to contain a thermal storageliquid at a storage pressure that is greater than atmospheric pressure,a liquid passage having an inlet connectable to a thermal storage liquidsource and configured to convey the thermal storage liquid to the liquidreservoir, and a first heat exchanger provided in the liquid inletpassage and in fluid communication between the first compression stageand the accumulator. Whereby when the compressed gas energy storagesystem is in the charging mode thermal energy is transferred from acompressed gas stream exiting the gas compressor/expander subsystem tothe thermal storage liquid.

The thermal storage liquid may be heated to a storage temperature priorto entering the first storage reservoir. The storage temperature isbelow a boiling temperature of the thermal storage liquid when at thestorage pressure and is the above boiling temperature of the thermalstorage liquid when at atmospheric pressure.

The storage temperature may be between about 150 degrees Celsius andabout 350 degrees Celsius.

A layer of compressed gas within the accumulator may be at anaccumulator pressure, and the storage pressure may be equal to orgreater than the accumulator pressure.

The storage pressure may be between about 100% and about 200% of theaccumulator pressure.

The storage pressure may be between about 20 bar and about 60 bar.

The first storage reservoir may include a pressurized layer of cover gasabove the thermal storage liquid.

The thermal storage liquid may be isolated from the layer of liquidwithin the accumulator to prevent mixing therebetween and furthercomprising a gas pressurization passage fluidly connecting the layer ofcompressed gas within the accumulator to the layer of cover gas, wherebypressurizing the accumulator pressurizes the first storage reservoir.

A flow regulator may be positioned in the gas pressurization passage andconfigured to permit gas to flow from accumulator to the first storagereservoir and to prevent gas from first storage reservoir to theaccumulator, so that the storage pressure can be higher than theaccumulator pressure.

A thermal storage compressor system may be configured to pressurize thelayer of cover gas to the storage pressure.

The layer of cover gas may be formed by the boiling of a portion of thethermal storage liquid within the first storage reservoir whereby thelayer of cover gas is pressurized to the storage pressure.

A thermal conditioning system may be in fluid communication with thelayer of cover gas, the thermal conditioning system operable to reducethe temperature of the layer of cover gas.

The first storage reservoir may be at least partially disposed withinthe accumulator.

The thermal storage liquid source may include a source reservoircontaining a quantity of the thermal storage liquid at a sourcetemperature that is less than the storage temperature.

The thermal storage liquid within the source reservoir may be at asource pressure that is greater than atmospheric pressure.

The source pressure may be substantially equal to the storage pressure.

The source reservoir may be external the first storage reservoir.

The thermal storage liquid in the first storage reservoir may beisolated from the quantity of thermal storage liquid in the sourcereservoir to prevent mixing therebetween and the source reservoir mayinclude a layer of cover gas above the quantity of thermal storageliquid, and further comprising a reservoir gas passage fluidlyconnecting the layer of cover gas within the first storage reservoir tothe layer of cover gas within the source reservoir, whereby the firststorage reservoir and source reservoir are maintained at the samepressure.

The source reservoir may include a body of water.

The source reservoir may be at least partially disposed within theaccumulator.

The first storage reservoir may be at least partially underground.

The gas compressor/expander system may include a second compressionstage downstream from the first compression stage and the first heatexchanger may be fluid communication between the first compression stageand the second compression stage. The thermal storage subsystem mayinclude a second heat exchanger in fluid communication between thesecond compression stage and the accumulator. Thermal energy may betransferred between the compressed gas stream exiting the secondcompression stage and the thermal storage liquid.

The gas compressor/expander system may include a third compression stagedownstream from the second compression stage and the second heatexchanger may be fluid communication between the second compressionstage and the third compression stage. The thermal storage subsystem mayinclude a third heat exchanger in fluid communication between the thirdcompression stage and the accumulator. Thermal energy may be transferredbetween the compressed gas stream exiting the third compression stageand the thermal storage liquid.

The first storage liquid reservoir may include a single chamber having achamber bottom wall, a chamber top wall, a chamber sidewall extendingtherefrom and may define a chamber interior configure to contain thethermal storage liquid.

The chamber may include a natural underground cavity formed at leastpartially of natural rock.

A storage liner may cover at least a portion of an interior surface ofthe chamber.

The compressed gas energy storage system of any one of claims 1 toError! Reference source not found., wherein the first storage reservoircomprises an outer chamber having a chamber upper wall, a chamber bottomwall, a chamber sidewall extending therefrom and defining a chamberinterior, and at least a first liquid tank having a tank bottom wall, atank sidewall extending therefrom and defining a tank interior, thefirst liquid tank being disposed within the interior of the chamber andconfigured to contain the thermal storage liquid.

The tank interior may be in fluid communication with the interior of thechamber whereby an internal pressure of the tank is substantiallyequalized with an internal pressure of the chamber.

An upper end of the tank may be at least partially open to provide thefluid communication with the interior of the chamber.

The tank may be formed at least partially from at least one of concreteand metal.

The tank bottom wall may be spaced above the chamber bottom wall and abottom thermal insulation layer may be positioned therebetween toinhibit heat transfer from the tank bottom wall to the chamber bottomwall.

The bottom thermal insulation layer may include at least one of a gaslayer, an insulating material layer, and a flowing cooling fluid layer.

The tank sidewall may be spaced apart from the chamber sidewall, and asidewall thermal insulation layer may be positioned therebetween toinhibit heat transfer from the tank sidewall to the chamber sidewall.

The sidewall thermal insulation layer may include at least one of a gaslayer, an insulating material layer, and a flowing cooling fluid layer.

An extraction pump may be in liquid communication with the thermalstorage liquid in the first storage reservoir and may be selectablyoperable to pump the thermal storage liquid at the storage temperatureout of the first storage reservoir.

An exit stream of gas is released from the accumulator, thermal energyis transferred from the thermal storage liquid pumped out of the firststorage reservoir into the exit stream of gas.

The exit stream of gas and the thermal storage liquid pumped out of thefirst storage reservoir may pass through the first heat exchanger.

The pump may include a progressive cavity pump having a rotor andcomplimentary stator disposed within the first storage reservoir. Amotor may be disposed external the first storage reservoir and a shaftmay drivingly connect the rotor to the motor.

The motor may be disposed above ground.

The first storage reservoir may be disposed entirely under ground.

A reservoir cooling system may be configured to selectably cool thetemperature of the thermal storage liquid contained in the first storagereservoir, thereby reducing the storage pressure within the firststorage reservoir.

The reservoir cooling system may include a quantity of a cooling liquidstored at a cooling temperature that is below the storage temperatureand may be operable to introduce the quantity of cooling liquid into thefirst storage reservoir, thereby diluting and reducing the temperatureof the thermal storage liquid contained in the first storage reservoir.

The reservoir cooling system may include an actuatable drain apparatusthat is openable to drain at least some of the thermal storage liquidfrom the first storage reservoir into a cooling chamber containing aquantity of a cooling liquid stored at a cooling temperature that isbelow the storage temperature.

The cooling chamber may be disposed at a lower elevation than the firststorage reservoir, whereby when the drain apparatus is opened thethermal storage liquid flows into the cooling chamber under theinfluence of gravity.

The drain apparatus may include a pressure-actuated drain valve that isoperable to open automatically when the storage pressure exceeds apredetermined automatic-cooling pressure threshold.

The accumulator may have a primary opening, an upper wall, a lower walland an accumulator interior containing a layer of the compressed gasabove a layer of water when in use and may be at least partially boundedthe upper wall and lower wall.

A shaft may have a lower end adjacent the primary opening, an upper endspaced apart from the lower end, and a shaft sidewall extending upwardlyfrom the lower end to the upper end and at least partially bounding ashaft interior for containing a quantity of a liquid, the shaft beingfluidly connectable to a liquid source/sink via a liquid supply conduit.

A partition may cover the primary opening and may separate theaccumulator interior from the shaft interior. The partition may have anouter surface in communication with the shaft interior and an opposinginner surface in communication with the accumulator interior.

At least one of the layer of compressed gas and the layer of liquid maybear against and exert an internal accumulator force on the innersurface of the partition and the quantity of liquid within the shaft maybear against and exert an external counter force on the outer surface ofthe partition, whereby a net force acting on the partition while thecompressed gas energy storage system is in use is a difference betweenthe accumulator force and the counter force and is less than theaccumulator force.

When the compressed gas energy storage system is in the discharging modecompressed gas may travel from the accumulator to the gascompressor/expander subsystem and at least a portion of the thermalstorage liquid at the storage temperature may be withdrawn from thefirst storage reservoir and the thermal storage subsystem may beoperable so that thermal energy is transferred from at least the portionof the thermal storage liquid withdrawn from the first storage reservoirto the compressed gas exiting the accumulator whereby the temperature ofthe compressed gas exiting the accumulator is increased before itreaches the gas compressor/expander subsystem.

When the compressed gas energy storage system is in a discharging modethe compressed gas traveling from the accumulator to the gascompressor/expander subsystem may pass through the first heat exchangerto receive thermal energy from the thermal storage liquid.

Other aspects and embodiments are described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theappended drawings in which:

FIG. 1 is a schematic, cross-sectional view of components of one exampleof a hydrostatically compensated compressed gas energy storage system;

FIG. 2 is a top plan view of components of a bulkhead for the compressedgas energy storage subsystem of FIG. 1 ;

FIG. 3 is a side elevation view of the bulkhead of FIG. 2 ;

FIG. 4 is a side cross-sectional view of the bulkhead of FIG. 2 , takenalong line 4-4;

FIG. 5 is a schematic representation of components of one example of acompressor/expander subsystem that is usable with any of the compressedgas energy storage systems, according to an embodiment.

FIG. 6 is a schematic, cross-sectional view of components of anotherexample of a compressed gas energy storage system;

FIG. 7 is a schematic, cross-sectional view of components of anotherexample of a compressed gas energy storage system;

FIG. 8 is a schematic view of components of a compressor/expandersubsystem for the compressed gas energy storage system, according to anembodiment;

FIG. 9 is a schematic view of components of an alternativecompressor/expander subsystem for a compressed gas energy storagesystem, with multiple compression stages each associated with arespective stage of a thermal storage subsystem;

FIG. 10 is a schematic view of components of an alternativecompressor/expander subsystem for a compressed gas energy storagesystem, with multiple expansion stages each associated with a respectivestage of a thermal storage subsystem;

FIG. 11 is a schematic view of components of an alternativecompressor/expander subsystem for a compressed gas energy storagesystem, with pairs of compression and expansion stages each associatedwith a respective stage of a thermal storage subsystem;

FIG. 12 is a schematic view of components of the alternativecompressor/expander subsystem of FIG. 11 , showing airflow during anexpansion (release) phase from storage through multiple expanders andrespective stages of a thermal storage subsystem;

FIG. 13 is a schematic view of components of the alternativecompressor/expander subsystem of FIG. 11 , showing airflow during acompression (storage) from the ambient through multiple compressors andrespective stages of a thermal storage subsystem;

FIG. 14 is a sectional view of components of a compressed gas energystorage system, according to an alternative embodiment;

FIG. 15 is a sectional view of components of an alternative compressedgas energy storage system, according to another alternative embodiment;

FIG. 16 is a schematic, cross-sectional view of components of anotherexample of a compressed gas energy storage system;

FIG. 17 is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system;

FIG. 18 is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system;

FIG. 19 is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system;

FIG. 20A is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system;

FIG. 20B is an enlarged view of a portion of the compressed gas energystorage system of FIG. 20A;

FIG. 21 is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system;

FIG. 22 is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system; and

FIG. 23 is a schematic, cross-sectional view of components of yetanother example of a compressed gas energy storage system.

DETAILED DESCRIPTION

Various apparatuses or processes will be described below to provide anexample of an embodiment of each claimed invention. No embodimentdescribed below limits any claimed invention and any claimed inventionmay cover processes or apparatuses that differ from those describedbelow. The claimed inventions are not limited to apparatuses orprocesses having all of the features of any one apparatus or processdescribed below or to features common to multiple or all of theapparatuses described below. It is possible that an apparatus or processdescribed below is not an embodiment of any claimed invention. Anyinvention disclosed in an apparatus or process described below that isnot claimed in this document may be the subject matter of anotherprotective instrument, for example, a continuing patent application, andthe applicants, inventors or owners do not intend to abandon, disclaimor dedicate to the public any such invention by its disclosure in thisdocument.

Energy produced by some types of energy sources, such as windmills,solar panels and the like may tend to be produced during certain periods(for example when it is windy, or sunny respectively), and not producedduring other periods (if it is not windy, or at night, etc.). However,the demand for energy may not always match the production periods, andit may be useful to store the energy for use at a later time. Similarly,it may be helpful to store energy generated using conventional powergenerators (coal, gas and/or nuclear power plants for example) to helpfacilitate storage of energy generated during non-peak periods (e.g.periods when electricity supply could be greater than demand and/or whenthe cost of electricity is relatively high) and allow that energy to beutilized during peak periods (e.g. when the demand for electricity maybe equal to or greater than the supply, and/or when the cost ofelectricity is relatively high).

As described herein, compressing and storing a gas (such as air), usinga suitable compressed gas energy storage system, is one way of storingenergy for later use. For example, during non-peak times, energy (i.e.electricity) can be used to drive compressors and compress a volume ofgas to a desired, relatively high pressure for storage. The gas can thenbe stored at the relatively high pressure inside any suitable containeror vessel, such as a suitable accumulator. To extract the stored energy,the pressurized gas can be released from the accumulator and used todrive any suitable expander apparatus or the like, and ultimately to beused to drive a generator or the like to produce electricity. The amountof energy that can be stored in a given compressed gas energy storagesystem may be related to the pressure at which the gas iscompressed/stored, with higher pressure storage generally facilitating ahigher energy storage. However, containing gases at relatively highpressures in conventional systems, such as between about 45-150 atm, canrequire relatively strong, specialized and often relatively costlystorage containers/pressure vessels.

Referring to FIG. 1 one example of a hydrostatically compensatedcompressed gas energy storage system 10, that can be used to compress,store and release a gas, includes an accumulator 12 that is locatedunderground (although in another embodiment the accumulator may belocated above ground). In this example, the accumulator 12 serves as achamber for holding both compressed gas and a liquid (such as water) andcan include any suitable type of pressure vessel or tank, or as in thisexample can be an underground cave or chamber that is within ground 200.In this embodiment, accumulator 12 is lined, for example using concrete,metal, plastic and combinations thereof or the like, to help make itsubstantially gas and/or liquid impermeable so as to help to preventunwanted egress of gas or liquid from within the interior 23. In anotherembodiment, the accumulator is preferably impermeable to gas and orliquid without requiring a lining.

The accumulator 12 may have any suitable configuration, and in thisexample, includes an upper wall 13 and an opposing lower wall 15 thatare separated from each other by an accumulator height 17. The upper andlower walls 13 and 15 may be of any suitable configuration, includingcurved, arcuate, angled, and the like, and in the illustrated exampleare shown as generally planar surfaces, that are generally parallel to ahorizontal reference plane 19. The accumulator 12 also has anaccumulator width (not shown—measured into the page as illustrated inFIG. 1 ). The upper and lower walls 13 and 15, along with one or moresidewalls 21 at least partially define an interior 23 of the accumulator12, that has an accumulator volume. The accumulator 12 in a givenembodiment of the system 10 can be sized based on a variety of factors(e.g. the quantity of gas to be stored, the available space in a givenlocation, etc.) and may, in some examples may be between about 1,000 m³and about 2,000,000 m³ or more. For example, in this embodiment theaccumulator 12 contains a layer of stored compressed gas 14 atop a layerof liquid 16, and its volume (and thus capacity) can be selected basedon the quantity of gas 14 to be stored, the duration of storage requiredfor system 10, and other suitable factors which may be related to thecapacity or other features of a suitable power source and/or power load(see power source/load S/L in FIG. 5 ) with which the system 10 is to beassociated. The power source/load S/L may be, in some examples, a powergrid, a power source (including renewable and optionally non-renewablesources) and the like.

Preferably, the accumulator 12 may be positioned below ground orunderwater, but alternatively may be at least partially above ground.Positioning the accumulator 12 within the ground 200, as shown, mayallow the weight of the ground/soil to help backstop/buttress the walls13, 15 and 21 of the accumulator 12, and help resist any outwardlyacting forces that are exerted on the walls 13, 15 and 21 of theinterior 23 of the accumulator. Its depth in the ground is establishedaccording to the pressures at which the compression/expansion equipmentto be used is most efficiently operated.

The gas that is to be compressed and stored in the accumulator 12 may beany suitable gas, including, but not limited to, air, nitrogen, noblegases and combinations thereof and the like. Using air may be preferablein some embodiments as a desired quantity of air may be drawn into thesystem from the surrounding, ambient environment and gas/air that isreleased from within the accumulator 12 can similarly be vented to theambient environment, optionally without requiring further treatment. Inthis embodiment, the compressed gas 14 is compressed atmospheric air,and the liquid is water.

Optionally, to help provide access to the interior of the accumulator12, for example for use during construction of the accumulator and/or topermit access for inspection and/or maintenance, the accumulator 12 mayinclude at least one opening that can be sealed in a generally air/gastight manner when the system 10 is in use. In this example, theaccumulator 12 includes a primary opening 27 that is provided in theupper wall 13. The primary opening 27 may be any suitable size, and mayhave a cross-sectional area (taken in the plane 19) that is adequatebased on the specific requirements. In one embodiment thecross-sectional area is between about 0.75 m² and about 80 m², but maybe larger or smaller in a given embodiment.

When the system 10 is in use, the primary opening 27 may be sealed usingany suitable type of partition that can function as a suitable sealingmember. In the embodiment of FIG. 1 , the system 10 includes a partitionin the form of a bulkhead 24 that covers the primary opening 27. FIG. 2is a top plan view of components of this embodiment of a bulkhead 24,and FIGS. 3 and 4 are side elevation and side cross-sectional views,respectively, of bulkhead 24. In this example, the bulkhead 24 has amain body 25 that includes a lower surface 29 that faces the interior 23of the accumulator 12, and in one alternative, is generally exposed toand in fluid communication with the compressed gas layer 14, and anopposing upper surface 31 at an upper end of the body 25 that facesinterior 54. A flange 26 extends generally laterally outwardly towardthe lower end of the bulkhead, such that the upper end of the bulkhead24 has an upper width 33 that may be between about 1-8 m, and may besized to fit within the opening 27, and the lower end of the bulkhead 24has a lower width 35 that is greater than the upper width 33 and can bebetween about 1.2 m and about 10 m, for example. In this arrangement, agenerally upwardly facing shoulder surface 37 is defined and extendsaround the periphery of the bulkhead 24. When the bulkhead 24 is inplace, as shown in FIG. 1 , the shoulder surface 37 can abut the uppersurface 13 of the accumulator 12, and can help resist upward movement ofthe bulkhead 24 through the opening 27. The bulkhead 24 may be securedto, and preferably sealed with the upper wall 13 using any suitablemechanism to help seal and enclose the interior 23. In otherembodiments, the bulkhead 24 may have a different, suitableconfiguration.

The bulkhead 24 may be manufactured in situ, or may be manufacturedoffsite, and may be made of any suitable material, including, concrete,metal, plastics, composites and the like. In the illustrated embodiment,the bulkhead 24 is assembled in situ at the interface between shaft 18and accumulator 12 of multiple pieces of reinforced concrete.

In the embodiment of FIG. 1 , the primary opening 27 is provided in theupper surface 13 of the accumulator 12. Alternatively, in otherembodiments the primary opening 27 and any associated partition may beprovided in different portions of the accumulator 12, including, forexample, on a sidewall (such as sidewall 21), in a lower surface (suchas lower surface 15) or other suitable location. The location of theprimary opening 27, and the associated partition, can be selected basedon a variety of factors including, for example, the soil and undergroundconditions, the availability of existing structures (e.g. if the system10 is being retrofit into some existing spaces, such as mines, quarries,storage facilities and the like), operating pressures, shaftconfigurations and the like. For example, some aspects of the systems 10described herein may be retrofit into pre-existing underground chambers,which may have been constructed with openings in their sidewalls, floorsand the like. Utilizing some of these existing formations may helpfacilitate construction and/or retrofit of the chambers used in thesystem, and may reduce or eliminate the need to form additional openingsin the upper surfaces of the chambers. Reducing the total number ofopenings in the accumulator may help facilitate sealing and may helpreduce the chances of leaks and the like.

When the primary opening 27 extends along the sidewall 21 of theaccumulator 12, it may be positioned such that is contacted by only thegas layer 14 (i.e. toward the top of the accumulator 12), contacted byonly the liquid layer 16 (i.e. submerged within the liquid layer 16 andtoward the bottom of the accumulator) and/or by a combination of boththe gas layer 14 and the liquid layer 16 (i.e. partially submerged andpartially non-submerged in the liquid). The specific position of thefree surface of the liquid layer 16 (i.e. the interface between theliquid layer 16 and the gas layer 14) may change while the system 10 isin use as gas is forced into (causing the liquid layer to drop) and/orwithdrawn from the accumulator (allowing the liquid level to rise).

As illustrated in the schematic representation in FIG. 16 , the primaryopening 27 is provided in the sidewall 15 of the accumulator 12, and thebulkhead 24 is positioned such that is generally partially submerged inthe liquid layer 16 and partially exposed to the gas layer 14 when thesystem 10F is in use. In this example, the gas supply conduit 22 passesthrough the bulkhead 24 and is arranged so that its lower end 62 islocated toward the top of the accumulator 12 so that it will remain incommunication with the gas layer 14, and fluidly isolated from theliquid layer 16, regardless of the level of the liquid within theaccumulator 12. Alternatively, the gas supply conduit 22 may bepositioned such that it does not pass through the bulkhead 24 when thesystem is configured in this manner. A thermal storage subsystem 120,including any of the embodiments described herein, can be used incombination with an accumulator 12 having this arrangement. One exampleof a suitable thermal storage subsystem 120 is illustrated in FIG. 16 .

In the embodiments of FIGS. 1 and 16 , the partition includes afabricated bulkhead 24 that is positioned to cover, and optionally sealthe primary opening 27 in the accumulator perimeter. Alternatively, inother embodiments, the partition may be at least partially formed fromnatural materials, such as rock and the like. For example, a suitablepartition may be formed by leaving and/or shaping portions of naturallyoccurring rock to help form at least a portion of the pressure boundarybetween the interior of the accumulator and the shaft. Such formationsmay be treated, coated or otherwise modified to help ensure they aresufficiently gas impermeable to be able to withstand the desiredoperating pressure differentials between the accumulator interior andthe shaft. This may be done, in some embodiments, by selectivelyexcavating the shaft 18 and accumulator 12 such that a portion of thesurrounding rock is generally undisturbed during the excavation andconstruction of the shaft 18 and accumulator 12. Alternatively, rock orother such material may be re-introduced into a suitable location withinthe accumulator 12 and/or shaft 18 after having been previouslyexcavated. This may help reduce the need to manufacture a separatebulkhead and install it within the system 10. In arrangements of thisnature, the primary opening 27 may be formed as an opening in a sidewall21 of the accumulator 12, or alternatively one side of the accumulator12 may be substantially open such that the primary opening 27 extendssubstantially the entire accumulator height 17, and forms substantiallyone entire side of the accumulator 12.

Referring to FIG. 17 , another embodiment of a compressed gas storagesystem 10G is configured with a partition that includes a projection200A, identified using cross-hatching in FIG. 17 , that is formed fromgenerally the same material as the surrounding ground 200. In thisexample, the system 10G need not include a separately fabricatedbulkhead 24 as shown in other embodiments. The system 10 in thisembodiment is configured so that the gas supply conduit 22 is spacedapart from the projection 200A and does not extend through thepartition. Instead, a separate shaft or bore can be provided toaccommodate the conduit 22. To help provide liquid communication betweenthe interior of the shaft 18 and the liquid layer 16, a liquid supplyconduit 40 can be provided to extend through the projection 200A or, asillustrated, at least some of the liquid supply conduit 40 can beprovided by a flow channel that passes beneath the projection 200A andfluidly connects the shaft 18 to the liquid layer 16, and in ends 64 and66 of the liquid supply conduit 40 can be the open ends of the passage.

Optionally, in such embodiments the gas supply conduit 22 may bearranged to pass through the partition/projection 200A as illustrated inFIG. 17 . In this arrangement (and in the embodiment shown in FIG. 16 ),the conduit 22 can be configured so that its end 62 is positioned towardthe upper side of the accumulator 12 to help prevent the liquid layer 16reaching the end 62. Alternatively, the gas supply conduit 22 need notpass through the partition, as schematically illustrated using dashedlines for alternative conduit 22. A thermal storage subsystem 120,including any of the embodiments described herein, can be used incombination with an accumulator 12 having this arrangement. One exampleof a suitable thermal storage subsystem 120 is illustrated in FIG. 17 .

Optionally, the system 10G may be arranged so that the gas supplyconduit 22 passes at least partially through the liquid supply conduit40. This may help reduce the number of openings that need to be providedin the partition/projection 200A. In the embodiment of FIG. 17 , anotheroptional arrangement of gas supply conduit 22 is shown using dashedlines and passes through the flow channel, from the shaft 18 into theinterior of the accumulator 12. In this arrangement, the gas supplyconduit 22 is nested in, and passes through the liquid supply conduit40, and passes beneath the projection 200A. Optionally, a configurationin which at least some of the gas supply conduit 22 is received within aportion of the liquid supply conduit 40 may also be utilized in otherembodiments of the system 10 (including those described and illustratedherein), including those in which both the liquid supply conduit 40 andgas supply conduit 40 pass through the partition.

When the accumulator 12 is in use, at least one of the pressurized gaslayer 14 and the liquid layer 16, or both, may contact and exertpressure on the inner-surface 29 of the bulkhead 24, which will resultin a generally outwardly, (upwardly in this embodiment) acting internalaccumulator force, represented by arrow 41 in FIG. 1 , acting on thebulkhead 24. The magnitude of the internal accumulator force 41 isdependent on the pressure of the gas 14 and the cross-sectional area(taken in plane 19) of the lower surface 29. For a given lower surface29 area, the magnitude of the internal accumulator force 41 may varygenerally proportionally with the pressure of the gas 14.

Preferably, an inwardly, (downwardly in this embodiment) acting forcecan be applied to the outer-surface 31 of the bulkhead 24 to help offsetand/or counterbalance the internal accumulator force 41. Applying acounter force of this nature may help reduce the net force acting on thebulkhead 24 while the system 10 is in use. This may help facilitate theuse of a bulkhead 24 with lower pressure tolerances than would berequired if the bulkhead 24 had to resist the entire magnitude of theinternal accumulator force 41. This may allow the bulkhead 24 berelatively smaller, lighter and less costly. This arrangement may alsohelp reduce the chances of the bulkhead 24 failing while the system 10is in use. Optionally, a suitable counter force may be created bysubjecting the upper surface 31 to a pressurized environment, such as apressurized gas or liquid that is in contact with the upper surface 31,and calibrating the pressure acting on the upper surface 31 (based onthe relative cross-sectional area of the upper surface 31 and thepressure acting on the lower surface 29) so that the resulting counterforce, shown by arrow 46 in FIG. 1 , has a desirable magnitude. In someconfigurations, the magnitude of the counter force 46 may be betweenabout 80% and about 99% of the internal accumulator force 41, and mayoptionally be between about 90% and about 97%, and may be about equal tothe magnitude of the internal accumulator force 41.

In the present embodiment, the system 10 includes a shaft 18 having alower end 43 that is in communication with the opening 27 in the upperwall 13 of the accumulator 12, and an upper end 48 that is spaced apartfrom the lower end 43 by a shaft height 50. At least one sidewall 52extends from the lower end 43 to the upper end 48, and at leastpartially defines a shaft interior 54 having a volume. In thisembodiment, the shaft 18 is generally linear and extends along agenerally vertical shaft axis 51, but may have other configurations,such as a linear or helical decline, in other embodiments. The upper end48 of the shaft 18 may be open to the atmosphere A, as shown, or may becapped, enclosed or otherwise sealed. In this embodiment, shaft 18 isgenerally cylindrical with a diameter 56 of about 3 metres, and in otherembodiments the diameter 56 may be between about 2 m and about 15 m ormore, or may be between about 5 m and 12 m, or between about 2 m andabout 5 m. In such arrangements, the interior 52 of the shaft 18 may beable to accommodate about 1,000-150,000 m³ of water.

In this arrangement, the bulkhead 24 is positioned at the interfacebetween the shaft 18 and the accumulator 12, and the outer surface 31(or at least a portion thereof) closes and seals the lower end 43 of theshaft 18. Preferably, the other boundaries of the shaft 18 (e.g. thesidewall 52) are generally liquid impermeable, such that the interior 54can be filled with, and can generally retain a quantity of a liquid,such as water 20. A water supply/replenishment conduit 58 can providefluid communication between the interior 54 of the shaft 18 and a watersource/sink 150 to allow water to flow into or out of the interior ofthe shaft 18 as required when the system 10 is in use. Optionally, aflow control valve 59 (as shown in FIG. 1 ) may be provided in the watersupply/replenishment conduit 58. The flow control valve 59 can be openwhile the system 10 is in use to help facilitate the desired flow ofwater between the shaft 18 and the water source/sink 150. Optionally,the flow control valve 59 can be closed to fluidly isolate the shaft 18and the water source/sink 150 if desired. For example, the flow controlvalve 59 may be closed to help facilitate draining the interior 54 ofthe shaft 18 for inspection, maintenance or the like.

The water source/sink 150 may be of any suitable nature, and mayinclude, for example a connection to a municipal water supply orreservoir, a purposely built reservoir, a storage tank, a water tower,and/or a natural body of water such as a lake, river or ocean,groundwater, or an aquifer. In the illustrated example, the watersource/sink 150 is illustrated as a lake. Allowing water to flow throughthe conduit 58 may help ensure that a sufficient quantity of water 20may be maintained with shaft 18 and that excess water 20 can be drainedfrom shaft 18. The conduit 58 may be connected to the shaft 18 at anysuitable location, and preferably is connected toward the upper end 48.Preferably, the conduit 58 can be positioned and configured such thatwater will flow from the source/sink 150 to the shaft 18 via gravity,and need not include external, powered pumps or other conveyingapparatus. Although the conduit 58 is depicted in the figures ashorizontal, it may be non-horizontal.

In this example, the water 20 in the shaft 18 bears against the outsideof bulkhead 24 and is thereby supported atop bulkhead 24. The amount ofpressure acting on the outer surface 31 of the bulkhead 24 in thisexample will vary with the volume of water 20 that is supported, whichfor a given diameter 56 will vary with the height 50 of the watercolumn. In this arrangement, the magnitude of the counter force 46 canthen be generally proportional to the amount of water 20 held in theshaft 18. To increase the magnitude of the counter force 46, more water20 can be added. To reduce the magnitude of the counter force 46, water20 can be removed from the interior 54.

The layer of stored compressed air 14 underlying bulkhead 24 serves,along with the technique by which bulkhead 24 is stably affixed to thesurrounding in the ground, in one alternative to surrounding stone inthe ground at the interface between accumulator 12 and shaft 18, tosupport bulkhead 24 and the quantity of liquid contained within shaft18.

Preferably, as will be described, the pressure at which the quantity ofwater 20 bears against bulkhead 24 and can be maintained so thatmagnitude of the counter force 46 is as equal, or nearly equal, to themagnitude of the internal accumulator force 41 exerted by the compressedgas in compressed gas layer 14 stored in accumulator 12. In theillustrated embodiment, operating system 10 so as to maintain a pressuredifferential (i.e. the difference between gas pressure inside theaccumulator 12 and the hydrostatic pressure at the lower end 43 of theshaft 18) within a threshold amount—an amount preferably between 0 and 4Bar, such as 2 Bar—the resulting net force acting on the bulkhead 24(i.e. the difference between the internal accumulator force 41 and thecounter force 46) can be maintained below a pre-determined threshold netforce limit. Maintaining the net pressure differential, and the relatednet force magnitude, below a threshold net pressure differential limitmay help reduce the need for the bulkhead 24 to be very large andhighly-reinforced, and accordingly relatively expensive. In alternativeembodiments, using a relatively stronger bulkhead 24 and/or installationtechnique for affixing the bulkhead 24 to the accumulator 12 may helpwithstand relatively higher pressure and net pressure differential, butmay be more expensive to construct and install, all other things beingequal. Furthermore, the height 17 of the accumulator 12 may be importantto the pressure differential: if the height 17 is about 10 metres, thenthe upward pressure on the bulkhead 24 will be 1 Bar higher than thedownward pressure on the bulkhead 24 from the water 20 in shaft 18.

Each of shaft 18 and accumulator 12 may be formed in ground 200 usingtechniques similar to those used for producing mineshafts and otherunderground structures.

To help maintain substantially equal outward and inward forces 41 and 46respectively on the bulkhead 24, the system 10 may be utilized to helpmaintain a desired differential in accumulator and shaft pressures thatis below a threshold amount. These pressures may be controlled by addingor removing gas from the compressed gas layer 14 accumulator 12 usingany suitable compressor/expander subsystem 100, and water can beconveyed between the liquid layer 16 and the water 20 in shaft 18.

In this embodiment, a gas conduit 22 is provided to convey compressedair between the compressed gas layer 14 and the compressor/expandersubsystem 100, which can convert compressed air energy to and fromelectricity. Similarly, a liquid conduit 40 is configured to conveywater between the liquid layer 16 and the water 20 in shaft 18. Eachconduit 22 and 40 may be formed from any suitable material, includingmetal, plastic and the like.

In this example, the gas conduit 22 has an upper end 60 that isconnected to the compressor/expander subsystem 100, and a lower end 62that is in communication with the gas layer 14. The gas conduit 22 is,in this example, positioned inside and extends within the shaft 18, andpasses through the bulkhead 24 to reach the gas layer 14. Positioningthe gas conduit 22 within the shaft 18 may eliminate the need to bore asecond shaft and/or access point from the surface to the accumulator 12.This position may also leave the gas conduit 22 generally exposed forinspection and maintenance, for example by using a diver or robot thatcan travel through the water 20 within the shaft 18 and/or by drainingsome or all of the water from the shaft 18. Alternatively, as shownusing dashed lines in FIG. 1 and in the embodiment of FIG. 17 , the gasconduit 22 may be external the shaft 18. Positioning the gas conduit 22outside the shaft 18 may help facilitate remote placement of thecompressor/expander subsystem 100 (i.e. it need not be proximate theshaft 18) and may not require the exterior of the gas conduit 22 (or itshousing) to be submerged in water. This may also eliminate the need forthe gas conduit 22 to pass through the partition that separates theaccumulator 12 from the shaft 18.

The liquid conduit 40 is, in this example, configured with a lower end64 that is submerged in the water layer 16 while the system 10 is in useand a remote upper end 66 that is in communication with the interior 54of the shaft 18. In this configuration, the liquid conduit 40 canfacilitate the exchange of liquid between the liquid layer 16 and thewater 20 in the shaft 18. As illustrated in FIG. 1 , the liquid conduit40 can pass through the bulkhead 24 (as described herein), oralternatively, as shown using dashed lines, may be configured to providecommunication between the liquid layer 16 and the water 20, but not pasthrough the bulkhead 24.

In this arrangement, as more gas is transferred into the gas layer 14during an accumulation cycle, and its pressure increases, in thisalternative slightly, water in the water layer 16 can be displaced andforced upwards through liquid conduit 40 into shaft 18 against thepressure of the water 20 in the shaft 18. More particularly, water canpreferably freely flow from the bottom of accumulator 12 and into shaft18, and ultimately may be exchanged with the source/sink 150 of water,via a replenishment conduit 58. Alternatively, any suitable type of flowlimiting or regulating device (such as a pump, valve, orifice plate andthe like) can be provided in the water conduit 40. When gas is removedfrom the gas layer 14, water can be forced from the shaft 18, throughthe water conduit 40, to refill the water layer 16. The flow through thereplenishment conduit 58 can help ensure that a desired quantity ofwater 20 may be maintained within shaft 18 as water is forced into andout of the water layer 16, as excess water 20 can be drained from andmake-up water can be supplied to the shaft 18. This arrangement canallow the pressures in the accumulator 12 and shaft 18 to at leastpartially, automatically re-balance as gas is forced into theaccumulator 12.

Preferably, the lower end 64 of the liquid conduit 40 is positioned sothat it is and generally remains submerged in the liquid layer 16 whilethe system 10 is in use, and is not in direct communication with the gaslayer 14. In the illustrated example, the lower wall 15 is planar and isgenerally horizontal (parallel to plane 19, or optionally arranged tohave a maximum grade of between about 0.01% to about 1%, and optionallybetween about 0.5% and about 1%, from horizontal), and the lower end 64of the liquid conduit 40 is placed close to the lower wall 15. If thelower wall 15 is not flat or not generally horizontal, the lower end 64of the liquid conduit 40 is preferably located in a relative low pointof the accumulator 12 to help reduce the chances of the lower end 64being exposed to the gas layer 14.

Similarly, to help facilitate extraction of gas from the gas layer, thelower end 62 of the gas conduit 22 is preferably located close to theupper wall 13, or at a relative high-point in the interior 23 of theaccumulator 12. This may help reduce material trapping of any gas in theaccumulator 12. For example, if the upper wall 13 were oriented on agrade, the point at which gas conduit 22 interfaces with the gas layer(i.e. its lower end 62) should be at a high point in the accumulator 12,to help avoid significant trapping of gas.

FIG. 5 is a schematic view of components of the compressor/expandersubsystem 100 for the compressed gas energy storage system 10 describedherein, according to an embodiment. In this example, thecompressor/expander subsystem 100 includes a compressor 112 of single ormultiple stages, driven by a motor 110 that is powered, in onealternative, using electricity from a power grid or by a renewable powersource or the like, and optionally controlled using a suitablecontroller 118. Compressor 112 is driven by motor 110 during anaccumulation stage of operation, and draws in atmospheric air A,compresses the air, and forces it down into gas conduit 22 for storagein accumulator 12 (via thermal storage subsystem 120 (see FIG. 6 forexample) in embodiments including same). Compressor/expander subsystem100 also includes an expander 116 driven by compressed air exiting fromgas conduit 22 during an expansion stage of operation and, in turn,driving generator 114 to generate electricity. After driving theexpander 116, the expanded air is conveyed for exit to the atmosphere A.While shown as separate apparatuses, the compressor 112 and expander 116may be part of a common apparatus, as can a hybrid motor/generatorapparatus. Optionally, the motor and generator may be provided in asingle machine.

Air entering or leaving compressor/expander subsystem 100 may beconditioned prior to its entry or exit. For example, air exiting orentering compressor/expander subsystem 100 may be heated and/or cooledto reduce undesirable environmental impacts or to cause the air to be ata temperature suited for an efficient operating range of a particularstage of compressor 112 or expander 116.

Controller 118 operates compressor/expander subsystem 100 so as toswitch between accumulation and expansion stages as required, includingoperating valves for preventing or enabling release of compressed airfrom gas conduit 22 on demand.

Optionally, the bulkhead 24 may include one or more apertures or othersuitable structures to accommodate the gas conduit 22, the liquidconduit 40 and other such conduits, such that the conduits pass throughthe bulkhead 24 to enter the interior 23 of the accumulator 12. Passingthe conduits and other such structures through the bulkhead 24 mayeliminate the need to make additional shafts/bores to reach theaccumulator 12, and may reduce the number of individual openingsrequired in the upper wall 13. Referring to FIGS. 2-4 , extendingthrough main body 25 is a first aperture 28 for accommodating passage ofgas conduit 22 from above bulkhead 24 in shaft 18 through to gas layer14 within accumulator 12. Gas conduit 22 is preferably sealed to/withinfirst aperture 28 to minimize, and preferably prevent, leaks or otheruncontrolled release of compressed gas within accumulator 12 into shaft18 or water 20 within shaft 18 into accumulator 12. Also extendingthrough bulkhead 24 is a second aperture 32 for accommodating passage ofliquid conduit 40 from above bulkhead 24 in shaft 18 through to liquidlayer 16 within accumulator 12. Liquid conduit 40 is sealed withinsecond aperture 32 to minimize, and preferably prevent, uncontrolledrelease of compressed gas within accumulator 12 into shaft 18 or water20 within shaft 18 into accumulator 12 (except via conduit 40).

In this embodiment, an openable and re-sealable access manway 30 isprovided for enabling maintenance access by maintenance personnel to theinterior of accumulator 12, for inspection and cleaning. This would bedone by closing flow control valve 59 (FIG. 1 ) and emptying shaft 18 ofliquid 20, and emptying accumulator 12 of compressed gas thereby toenable manway 30 to be opened and personnel to pass back and forth. Asfor bulkhead 24, variations are possible. For example, in an alternativeembodiment, bulkhead 24 may only have first and second apertures 28, 32but no manway 30. In an alternative embodiment, bulkhead 24 may includea manway 30, but need not contain first and second apertures 28, 32 andthe conduits 22 and 40 do not pass through bulkhead 24. In yet anotheralternative embodiment, bulkhead 24 contains no manway and no apertures,such that fluid communication with accumulator 12 does not pass throughbulkhead 24. Optionally, a manway or the like may also be provided inother types of partitions, including for example the projection 200A asshown in the embodiment of FIG. 17 .

Optionally, some embodiments of the compressed gas energy storage systemmay include a thermal storage subsystem that can be used to absorb heatfrom the compressed gas that is being directed into the accumulator 12(i.e. downstream from the compressor 112), sequester at least a portionof the thermal energy for a period, and then, optionally, release atleast a portion of the sequestered heat back into gas that is beingextracted/released from the accumulator 12 (i.e. upstream from theexpander 116). In such examples, the gas may exit thecompressor/expander subsystem 100, after being compressed, at an exittemperature of between about 180° C. and about 300° C. and may be cooledby the thermal storage subsystem to an accumulator temperature that isless than the exit temperature, and may be between about 30° C. andabout 60° C. in some examples.

FIG. 6 is a schematic view of components of a compressed gas energystorage system 10A, according to an alternative embodiment. Compressedgas energy storage system 10A is like compressed gas energy storagesystems 10, with the addition of a thermal storage subsystem 120 that isprovided in the gas flow path between the compressor/expander subsystem100 and the accumulator 12. In this example, the gas conduit 22 thatconveys the compressed gas between the compressed gas layer 14 andcompressor/expander subsystem 100 includes an upper portion 22A thatextends between the compressor/expander subsystem 100 and thermalstorage subsystem 120, and a lower portion 22B that extends betweenthermal storage subsystem 120 and accumulator 12.

The thermal storage subsystem 120 may include any suitable type ofthermal storage apparatus, including, for example latent and/or sensiblestorage apparatuses. The thermal storage apparatus(es) may be configuredas single stage, two stage and/or multiple stage storage apparatus(es).Similarly, the thermal storage subsystem 120 may include one or moreheat exchangers (to transfer thermal energy into and/or out of thethermal storage subsystem 120) and one or more storage apparatuses(including, for example storage reservoirs for holding thermal storagefluids and the like). Any of the thermal storage apparatuses may beeither be separated from or proximate to their associated heat exchangerand may also incorporate the associated heat exchanger in a singlecompound apparatus (i.e. in which the heat exchanger is integratedwithin the storage reservoir).

The thermal storage subsystem 120, or portions thereof, may be locatedin any suitable location, including above-ground, below ground, withinthe shaft 18, within the accumulator 12, and the like. Optionally,portions of the thermal storage subsystem 120 can be spaced apart fromeach other and located in different locations. For example, a heatexchanger used in a thermal storage subsystem 120 may be spaced apartfrom (but fluidly connected to) a corresponding storage apparatus. Insuch examples, the storage apparatus(es) may be located relatively deepwithin the ground while the heat exchanger may be relatively shallowerand/or may be provided above ground to help facilitate access, etc.

In the illustrated embodiment, substantially the thermal storagesubsystem 120 is located underground, which may help reduce the use ofabove-ground land and may help facilitate the use of the weight of theearth/rock to help contain the pressure in the storage reservoir. Thatis, the outward-acting pressure within the storage reservoir can besubstantially balanced by the inwardly-acting forces exerted by theearth and rock surrounding the first reservoir. In some examples, if aliner or other type of vessel are provided in the storage reservoir suchstructures may carry some of the pressure load, but are preferablybacked-up by and/or supported by the surrounding earth/rock. This canhelp facilitate pressurization of the storage reservoir to the desiredstorage pressures, without the need for providing a manufacturedpressure vessel that is capable of withstanding the entire pressuredifferential. In this example, the thermal storage subsystem 120 alsoemploys multiple stages including, for example, multiple sensible and/orlatent thermal storage stages such as stages having one or more phasechange materials and/or pressurized water, or other heat transfer fluidarranged in a cascade. It will be noted that, if operating the systemfor partial storage/retrieval cycles, the sizes of the stages may besized according to the time cycles of the phase change materials so thatthe phase changes, which take time, take place effectively within therequired time cycles.

In general, as gas is compressed by the compressor/expander subsystem100 during an accumulation cycle and is conveyed for storage towardsaccumulator 12, the heat of the compressed gas can be drawn out of thecompressed gas and into the thermal storage subsystem 120 for sensibleand/or latent heat storage. In this way, at least a portion of the heatenergy is saved for future use instead of, for example being leached outof the compressed gas into water 20 or in the liquid layer 16, andaccordingly substantially lost (i.e. non-recoverable by the system 10).

Similarly, during an expansion cycle as gas is released from accumulator12 towards compressor/expander subsystem 100 it can optionally be passedthrough thermal storage subsystem 120 to re-absorb at least some of thestored heat energy on its way to the expander stage of thecompressor/expander subsystem 100. Advantageously, the compressed gas,accordingly heated, can reach the compressor/expander subsystem 100 at adesired temperature (an expansion temperature—that is preferablywarmer/higher than the accumulator temperature), and may be within about10° C. and about 60° C. of the exit temperature in some examples, thatmay help enable the expander to operate within its relatively efficientoperating temperature range(s), rather than having to operate outside ofthe range with cooler compressed gas.

In some embodiments, the thermal storage subsystem 120 may employ atleast one phase change material, preferably multiple phase changematerials, multiple stages and materials that may be selected accordingto the temperature rating allowing for the capture of the latent heat.Generally, phase change material heat can be useful for storing heat ofapproximately 150 degrees Celsius and higher. The material is fixed inlocation and the compressed air to be stored or expanded is flowedthrough the material. In embodiments using multiple cascading phasechange materials, each different phase change material represents astorage stage, such that a first type of phase change material maychange phase thereby storing the heat at between 200 and 250 degreesCelsius, a second type of phase change material may change phase therebystoring the heat at between 175 and 200 degree Celsius, and a third typeof phase change material may change phase thereby storing the heat atbetween 150 and 175 degrees Celsius. One example of a phase changematerial that may be used with some embodiments of the system includes aeutectic mixture of sodium nitrate and potassium nitrate, or the HITEC®heat transfer salt manufactured by Coastal Chemical Co. of Houston,Texas.

In embodiments of the thermal storage subsystem 120 employing sensibleheat storage, pressurized water, or any other suitable thermal storagefluid/liquid and/or coolant, may be employed as the sensible heatstorage medium. Optionally, such systems may be configured so that thethermal storage liquid remains liquid while the system is in use, anddoes not undergo a meaningful phase change (i.e. does not boil to becomea gas). For example, such thermal storage liquids (e.g. water) may bepressurized and maintained at an operating pressure that is sufficientto generally keep the water in its liquid phase during the heatabsorption process as its temperature rises. Optionally, the pressurizedwater may be passed through a heat exchanger or series of heatexchangers to capture and return the heat to and from the gas streamthat is exiting the accumulator, via conduit 22. Generally, sensibleheat storage may be useful for storing heat of temperatures of 100degrees Celsius and higher. Pressurizing the water in these systems mayhelp facilitate heating the water to temperatures well above 100 degreesCelsius (thereby increasing its total energy storage capability) withoutboiling.

Optionally, in some embodiments, a thermal storage subsystem 120 maycombine both latent and sensible heat storage stages, and may use phasechange materials with multiple stages or a single stage. Preferably,particularly for phase change materials, the number of stages throughwhich air is conveyed during compression and expansion may be adjustableby controller 118. This may help the system 10 to adapt its thermalstorage and release programme to match desired and/or required operatingconditions.

Optionally, at least some of the gas conduit 22 may be external theshaft 18 so that it is not submerged in the water 20 that is held in theshaft 18. In some preferred embodiments, the compressed gas stream willtransfer its thermal energy to the thermal storage system 120 (forexample by passing through heat exchangers 635 described herein) beforethe compressed gas travels underground. That is, some portions of thethermal storage subsystem 120 and at least the portion of the gasconduit that extends between the compressor/expander subsystem 100 andthe thermal storage subsystem 120 may be provided above ground, as itmay be generally desirable in some embodiments to transfer as muchexcess heat from the gas to the thermal storage subsystem 120, andreduce the likelihood of heat being transferred/lost in the water 20,ground or other possible heat sinks along the length of the gas conduit22. Similar considerations can apply during the expansion stage, as itmay be desirable for the warmed gas to travel from the thermal storagesubsystem 120 to the compressor/expander subsystem 100 at a desiredtemperature, and while reducing the heat lost in transit.

Referring to FIG. 18 , one example of the thermal storage subsystem 120that can be used to transfer thermal energy from the compressed gasstream travelling between the gas compressor/expander subsystem 100 andthe accumulator 12 is configured to store thermal energy in a thermalstorage liquid 600. Optionally, the thermal storage liquid 600 can bepressurized in the thermal storage subsystem 120 to a storage pressurethat is higher than atmospheric pressure and may optionally be generallyequal to or greater than the accumulator pressure. Harmonizing thestorage pressure in the thermal storage subsystem 120 and theaccumulator 12 may help facilitate configurations in which there is atleast some fluid communication between the thermal storage subsystem 120and the accumulator 12 (including those described herein). In someexamples, the storage pressure may be between about 100% and about 200%of the accumulator pressure.

Pressurizing the thermal storage liquid 600 in this manner may allow thethermal storage liquid 600 to be heated to relatively highertemperatures (i.e. store relatively more thermal energy and at a morevaluable grade) without boiling, as compared to the same liquid atatmospheric pressure. That is, the thermal storage liquid 600 may bepressurized to a storage pressure and heated to a thermal storagetemperature such that the thermal storage liquid 600 is maintained as aliquid while the system is in use (which may help reduce energy lossthrough phase change of the thermal storage liquid). In the embodimentsillustrated, the storage temperature may be between about 150 and about500 degrees Celsius, and preferably may be between about 150 and 350degrees Celsius. The storage temperature is preferably below a boilingtemperature of the thermal storage liquid 600 when at the storagepressure but may be, and in some instances preferably will be the aboveboiling temperature of the thermal storage liquid 600 if it were atatmospheric pressure. In this example, the thermal storage liquid 600can be water, but in other embodiments may be engineered heattransfer/storage fluids, coolants, oils and the like. When sufficientlypressurized, water may be heated to a storage temperature of about 250degrees Celsius without boiling, whereas water at that temperature wouldboil at atmospheric pressure.

Optionally, the thermal storage liquid 600 can be circulated through asuitable heat exchanger to receive heat from the compressed gas streamtravelling through the gas supply conduit 22 (downstream from thecompressor/expander subsystem 100). The heated thermal storage liquid600 can then be collected and stored in a suitable storage reservoir (ormore than one storage reservoirs) that can retain the heated thermalstorage liquid 600 and can be pressurized to a storage pressure that isgreater than atmospheric pressure (and may be between about 10 and 60bar, and may be between about 30 and 45 bar, and between about 20 and 26bar).

The storage reservoir may be any suitable type of structure, includingan underground chamber/cavity (e.g. formed within the surrounding ground200) or a fabricated tank, container, a combination of a fabricated tankand underground chamber/cavity, or the like. If configured to include anunderground chamber, the chamber may optionally be lined to help providea desired level of liquid and gas impermeability and/or thermalinsulation. For example, underground chambers may be at least partiallylined with concrete, polymers, rubber, plastics, geotextiles, compositematerials, metal and the like. Configuring the storage reservoir to beat least partially, and preferably at least substantially impermeablemay help facilitate pressurizing the storage reservoir as describedherein. Fabricated tanks may be formed from any suitable material,including concrete, metal, plastic, glass, ceramic, composite materialsand the like. Optionally, the fabricated tank may include concrete thatis reinforced using, metal, fiber reinforced plastic, ceramic, glass orthe like, which may help reduce the thermal expansion difference betweenthe concrete and the reinforcement material.

Referring still to FIG. 18 , in this embodiment the storage reservoir610 of the thermal storage subsystem 120 includes a chamber 615 that ispositioned underground, at a reservoir depth 660. Preferably, thereservoir depth 660 is less than the depth of the accumulator 12, whichin this example corresponds to the shaft height 50. Optionally, thethermal storage subsystem 120 can be configured so that the reservoirdepth 660 is at least about ⅓ of the accumulator depth/shaft height 50,or more. For example, if the accumulator 12 is at a depth of about 300m, the reservoir depth 660 is preferably about 100 m or more. Forexample, having the reservoir depth 660 being less than the accumulatordepth 50 may help facilitate sufficient net positive suction head to beavailable to the fluid transfer pumps and other equipment utilized topump the thermal storage liquid 600 through the thermal storagesubsystem 120 (for example between source reservoir 606 and storagereservoir 610). This may allow the transfer pumps to be positionedconveniently above ground and may help reduce the chances of damagingcavitation from occurring.

The reservoir depth 660 being at least ⅓ the depth 50 of the accumulator12 may also allow for relatively higher rock stability of thesubterranean thermal storage cavern, such as chamber 615. The geostaticgradient, which provides an upper limit on pressure inside undergroundrock caverns, is typically about 2.5-3 times the hydrostatic gradient.Given this rock stability criterion, the shallowest reservoir depth 660may be approximately three times less than the accumulator depth in someembodiments, such as when the storage pressure is generally equal thanthe accumulator pressure.

In this example, the chamber 615 is a single chamber having a chamberinterior 616 that is at least partially defined by a bottom chamber wall620, a top chamber wall 651, and a chamber sidewall 621. The chamber 615is connected to one end of a liquid inlet passage 630 (such as a pipe orother suitable conduit) whereby the thermal storage liquid 600 can betransferred into and/or out of the chamber 615. In addition to the layerof thermal storage liquid 600, a layer of cover gas 602 is contained inthe chamber 615 and overlies the thermal storage liquid 600. Like thearrangement used for the accumulator 12, the layer of cover gas 602 canbe pressurized using any suitable mechanism to help pressurize theinterior of the chamber 615 and thereby help pressurize the thermalstorage liquid 600. The cover gas may be any suitable gas, includingair, nitrogen, thermal storage liquid vapour, an inert gas and the like.Optionally, at least the subterranean portions of the liquid inletpassage 630 (i.e. the portions extending between the heat exchanger 635and the storage reservoir 610) may be insulated (such as by a vacuumsleeve, or insulation material) to help reduce heat transfer between thethermal storage fluid and the surrounding ground.

When the thermal storage subsystem 120 is in use, a supply of thermalstorage liquid can be provided from any suitable thermal storage liquidsource 605. The thermal storage liquid source can be maintained at asource pressure that may be the same as the storage pressure, or may bedifferent than the storage pressure. For example, the thermal storageliquid source may be at approximately atmospheric pressure, which mayreduce the need for providing a relatively strong, pressure vessel forthe thermal storage liquid source. Alternatively, the thermal storageliquid source may be pressurized. The thermal storage liquid source mayalso be maintained at a source temperature that is lower, and optionallysubstantially lower than the storage temperature. For example, thethermal storage liquid source may be at temperatures of between about 2and about 100 degrees Celsius, and may be between about 4 and about 50degrees Celsius. Increasing the temperature difference between theincoming thermal storage liquid from the source and the storagetemperature may help increase the amount of heat and/or thermal energythat can be stored in the thermal storage subsystem 120.

The thermal storage liquid source 605 may have any suitableconfiguration, and may have the same construction as an associatedstorage reservoir, or may have a different configuration. For example,in the embodiment of FIG. 18 the thermal storage liquid source 605includes a source reservoir 606 that is configured in the sameunderground chamber as the thermal fluid storage chamber 615. In thisarrangement, a closed loop system can be provided, including the storagereservoir 610 and the source reservoir 606. Alternatively, as shown inthe embodiment of FIG. 19 , the thermal storage liquid source 605 mayinclude a source reservoir 606 that is configured as an above-groundvessel, and optionally need not be pressurized substantially aboveatmospheric pressure. In other embodiments, the thermal liquid source605 may include a body of water such as the lake 150, water 20 from theshaft 18, liquid from the liquid layer 16 in the accumulator 12 (or fromany other portion of the overall system 10), water from a municipalwater supply or other such sources and combinations thereof.

In the embodiment of FIG. 18 , the source reservoir 606 and storagereservoir 610 are adjacent each other, and are portions of a generallycommon underground chamber. This may help simplify construction of thethermal storage subsystem 120 as an excavation of a single chamber mayprovide space for both the source reservoir 606 and storage reservoir610. This may also help simplify piping and valving between the sourcereservoir 606 and the storage reservoir 610.

In some examples, the interiors of the storage reservoir 610 and sourcereservoir 606 may be substantially fluidly isolated from each other,such that neither gas nor liquid can easily/freely pass betweenreservoirs 606 and 610. One example of a subsystem 120 having thisarrangement is shown in FIG. 19 .

Alternatively, as illustrated in FIG. 18 , the interiors of the storagereservoir 610 and source reservoir 606 may be in gas flow communicationwith each other, such as by providing the gas exchange passage 626 thatcan connect the layer of cover gas 602 with a layer of cover gas 608 inthe source reservoir 606. The gas exchange passage 626 can be configuredto allow free, two-way flow of gas between the storage reservoir 610 andthe source reservoir 606, or may be configured to only allow one-way gasflow (in either direction). Providing a free flow of gas between thestorage reservoir 610 and the source reservoir 606 may helpautomatically match the pressures within the storage reservoir 610 andthe source reservoir 606. Preferably, when arranged in this manner, theinterior of the storage reservoir 610 remains at least partiallyisolated from the interior of the source reservoir 606 during normaloperation to inhibit, and preferably prevent mixing of the relativelyhot cover gas 602 associated with the thermal storage liquid 600 in thestorage reservoir 610 with the relatively cooler cover gas 608associated with the thermal storage liquid in the source reservoir 606.In this example, the storage reservoir 610 and source reservoir 606share a common sidewall, which can function as an isolating barrier 625to prevent liquid mixing between the reservoirs. This common sidewallmay be insulated to prevent unwanted heat transfer from the relativelyhot thermal storage liquid 600 in the storage reservoir 610 to therelatively cooler thermal storage liquid in the source reservoir 606

When the compressed gas energy storage system 10H is in a charging mode,compressed gas is being directed into the accumulator 12 and the thermalstorage liquid 600 can be drawn from the thermal storage liquid source605, passed through one side of a suitable heat exchanger 635 (includingone or more heat exchanger stages) to receive thermal energy from thecompressed gas stream exiting the compressor/expander subsystem 100, andthen conveyed/pumped through the liquid inlet passage 630 and into thestorage reservoir 610 for storage at the storage pressure.

When the compressed gas energy storage system is in a storage mode,compressed gas is neither flowing into or out of the accumulator 12 orthorough the heat exchanger 635, and the thermal storage liquid 600 neednot be circulated through the heat exchanger 635.

When the compressed gas energy storage system 10H is in a dischargingmode, compressed gas is being transferred from the accumulator 12 andinto the compressor/expander subsystem 100 for expansion and the thermalstorage liquid 600 can be drawn from the storage reservoir 610, passedthrough one side of a suitable heat exchanger 635 (including one or moreheat exchanger stages) to transfer thermal energy from thermal storageliquid into the compressed gas stream to help increase the temperatureof the gas stream before it enters the compressor/expander subsystem100. Optionally, the thermal storage fluid can then be conveyed/pumpedinto the source reservoir 606 for storage.

When the compressed gas energy storage system 10I is in charging modethe thermal storage liquid 600 receives thermal energy from thecompressed gas is conveyed into the storage reservoir 610, and while thethermal storage system 10I is in discharging mode the storage liquid 600is drawn from the storage reservoir 600 and transfers thermal energyinto the compressed gas exiting the accumulator 12 (preferably before itreaches the compressor/expander subsystem 100).

The thermal storage liquid 600 can be conveyed through the variousportions of the thermal storage subsystem 120 using any suitablecombination of pumps, valves, flow control mechanisms and the like.Optionally, an extraction pump may be provided in fluid communicationwith, and optionally at least partially nested within, the storagereservoir 610 to help pump the thermal storage liquid 600 from thestorage reservoir 610 up to the surface. Such a pump may be asubmersible type pump and/or may be configured so that the pump and itsdriving motor are both located within the storage reservoir 610.Alternatively, the pump may be configured as a progressive cavity pumphaving a stator and rotor assembly 668 (including a rotor rotatablyreceived within a stator) provided in the storage reservoir 610 andpositioned to be at least partially submerged in the thermal storageliquid 600, a motor 670 that is spaced from the stator and rotorassembly 668 (on the surface in this example) and a drive shaft 672extending therebetween. In this example, the drive shaft 672 is nestedwithin the liquid inlet passage 630 extending to the storage reservoir610, but alternatively may be in other locations.

Optionally, to help pressurize the storage reservoir 610, the thermalstorage subsystem 120 may include any suitable type of pressurizationsystem, and may include a thermal storage compressor system that canhelp pressurize the layer of cover gas 602 in the storage reservoir.This may include a thermal storage compressor 664, as shown in in FIGS.18 and 19 for example,) that is in fluid communication with the covergas layer 602. The compressor itself may be on the surface, and may beconnected to the cover gas layer 602 by a compressor gas conduit 666that may be spaced from, or at least partially integrated with theliquid inlet passage 630. Optionally, the compressor 664 may beconfigured to raise the pressure of the cover gas layer 602 fromatmospheric pressure to the storage pressure. The compressor 664, andany other aspects of the thermal storage subsystem 120 may be controlledat least partially automatically by the controller 118. While shown as aseparate compressor 664, pressure for the storage reservoir 610 may atleast partially be provided by the compressor/expander subsystem 100.

Optionally, as shown in the examples of FIGS. 19 and 21 , the cover gaslayer 602 may be in fluid communication with the compressed gas layer 14in the accumulator 12, for example via the gas exchange passage 626. Insuch examples, pressuring the accumulator 12 can also cause thesimultaneous pressurization of the storage reservoir 610, and raise thepressure of the cover gas layer 602 to the accumulator pressure. Inembodiments where the storage reservoir 610 is to be pressurized to thesame pressure as the accumulator, this may be sufficient pressurizationof the storage reservoir 610.

Alternatively, if the storage pressure is to be higher than theaccumulator pressure, the thermal storage subsystem 120 may include avalve, one-way flow control device or other such flow limiting devicethat can allow gas to move from the accumulator 12 into the storagereservoir 610 to pressurize the storage reservoir 610 to the accumulatorpressure, and can prevent gas from travelling from escaping from thestorage reservoir 610 to the accumulator 12. This may allow the storagereservoir 610 to be at least partially pressurized by the gas layer 14of the accumulator 12, and then isolated and further pressurized using asuitable pressurization system (such as the compressor 664).

In other embodiments, the storage reservoir 610, and cover gas layer 602therein, may be pressurized using other means, including, othermechanical compression mechanisms and may optionally be at leastpartially self pressurizing. That is, the storage reservoir 610 maybegin at relatively low pressure and as the thermal storage liquid 600is heated a relatively small portion of the thermal storage liquid 600may boil and convert to a vapour phase. The vapour may then form atleast part of the cover gas layer 602, and may increase the pressurewithin the storage reservoir 610 to a generally equilibrium pressuresuch that further boiling, at a given temperature, is inhibited. As thetemperature of the thermal storage liquid 600 continues to rise,additional amounts of the thermal storage liquid 600 may convert tovapour phase thereby increasing the overall pressure of the storagereservoir 610 and reaching a new equilibrium with the liquid phase. Thismay be sufficient to pressurize the storage reservoir 610 to the storagepressure, or the subsystem 120 may also include one or more additionalpressurization systems, including any of those described herein.

In the example of FIG. 19 , the thermal storage liquid source 605, e.g.the source reservoir 606 is located above ground and storage reservoir610 is located underground and is adjacent to the accumulator 12. Inthis arrangement, the depth of the storage reservoir 610 is the same asthe depth of the accumulator 12. To keep the thermal storage liquid 600separate from the liquid layer 16, there is an isolating barrier 625.Optionally, the chamber interior 616 may be at least partially coveredin a storage liner 617 that is preferably substantially vapour andliquid impermeable at the storage pressure.

In this embodiment, the isolating barrier 625 includes a gas exchangepassage 626 that allows the pressurized layer of cover gas 602 tocommunicate with gas layer 14 within the accumulator 12, which allowsthe mixing of gas 602 and gas layer 14 which allows the storagereservoir 610 to be at least partially pressurized when the accumulator12 is pressurized. Optionally, fluid communication through the gasexchange passage 626 can be directionally controlled by a flow regulator628 (e.g. a check valve) such that, for example, pressurized layer ofcover gas 602 cannot enter the accumulator 12 through the gas exchangepassage 626, but gas 14 from the accumulator 12 can enter the chamber615 of the storage reservoir 610 allowing the gas 14 in the accumulatorto initially pressurize the first storage reservoir 610. This may allowthe storage reservoir 610 to be at least partially pressurized by thegas layer 14 of the accumulator 12, and then isolated and furtherpressurized using a suitable pressurization system (such as thecompressor 664).

In this embodiment, the liquid inlet passage 630 includes an upperliquid inlet passage 629 and a lower liquid inlet passage 631. When thecompressed energy storage system 10I is in a charging mode, the upperliquid inlet passage 629 conveys thermal storage liquid 600 from thesource reservoir 606 to a first heat exchanger 635 where it is heated toa storage temperature (below a boiling temperature of the thermalstorage liquid when at the storage pressure and is the above boilingtemperature of the thermal storage liquid when at atmospheric pressure)before the lower liquid inlet passage 631 conveys the thermal storageliquid heated to a storage temperature to the first storage reservoir610. For greater certainty only, the thermal storage fluid 600 in thesource reservoir 606 is at a source temperature that is less than thestorage temperature described above. An analogous configuration may beused in other embodiments.

When the compressed gas energy storage system 10I is in a dischargingmode, compressed gas is being transferred from the accumulator 12 andinto the compressor/expander subsystem 100 for expansion and the thermalstorage liquid 600 can be drawn from the storage reservoir 610, passedthrough one side of a suitable heat exchanger 635 (including one or moreheat exchanger stages) to transfer thermal energy from thermal storageliquid into the compressed gas stream to help increase the temperatureof the gas stream before it enters the compressor/expander subsystem100, as illustrated by arrow 632 Optionally, the thermal storage fluidcan then be conveyed/pumped into the source reservoir 606 for storage.

Optionally, in some embodiments the storage reservoir 610 may include anouter chamber or shell portion that is configured to withstand thedesired pressurization described herein, and at least one inner chamberthat is configured to receive and retain the heated thermal storageliquid, and optionally may include two or more inner chambers within acommon outer chamber. In some examples, the interior of the innerchamber may be in fluid communication with the interior of the outerchamber. This may allow the inner chamber to retain the thermal transferfluid without having to be a pressure-bearing vessel or otherwise carrya substantial pressure differential across the boundary of the innerchamber. For example, the outer chamber may be a chamber formed in theground. Such a chamber may be strong enough to withstand the intendedoperating pressures of the thermal storage system 120, but may not bethe preferred configuration for directly contacting and retaining thethermal storage fluid. To help provide the desired liquid storage, aninner chamber in the form of a tank or other liquid retaining vessel maybe positioned inside the outer chamber. The heated thermal storageliquid can then be stored in the tank, under an associated cover gaslayer. The upper end of the tank may be at least partially open, suchthat the cover gas layer in the inner chamber is in communication with,and is therefore at the same pressure as the cover gas layer of theouter chamber. In this arrangement, the inner tank need not carry asubstantial pressure load (simply the hydrostatic pressure exerted bythe quantity of thermal storage liquid in the tank), and therefore maybe of relatively light construction, as compared to a pressure-bearingvessel that would be required to withstand the storage pressure. In someexamples, two or more separate tanks may be placed within a common outerchamber, and may be maintained at a common pressure in this manner.

Referring to FIGS. 20A and 20B, in this example the storage reservoir610 includes an outer chamber 615 that has a chamber interior 616 thatis at least partially defined by a bottom chamber wall 620, an upperchamber wall 651, and a chamber sidewall 621. An inner chamber includesa tank 684 that is disposed within the chamber interior 616, andincludes a tank bottom wall 686 and a tank sidewall 688 which togetherhelp define a tank interior 690. The thermal storage liquid 600 andlayer of cover gas 602 are contained within the tank 684. The upper endof the tank 684 is open in this example, providing fluid communicationbetween the chamber interior 616 and cover gas layer 602. The tank 684may be made from any suitable material, including, for example, metal,concrete, plastic, glass, ceramic, composite materials and combinationsthereof. The tank 684 is preferably liquid impermeable, but need not bevapour impermeable.

In this arrangement the storage pressure is carried by the relativelystrong walls 620, 651, and 621 of the outer chamber 615, and optionallythe tank 684 need not be strong enough to withstand the full storagepressure.

Optionally, the tank bottom wall 686 can be spaced above the chamberbottom wall 620 by a offset height 692. Similarly, the tank side wall688 may be spaced inwardly from the chamber sidewall 621 by an offsetdistance 694. This may help provide thermal insulation of the tank 684by surrounding it with gas, and may allow the tank 684 to have a desiredshape that can be different than the shape/contour of the chamber bottomwall 620 and chamber sidewall 621. The offset height and distance 692and 694 may be any suitable distance, and may be between 10 cm and about10 m or more.

Optionally, the thermal storage subsystem 120 may be configured toprovide at least some degree of thermal insulation between the heatedthermal storage liquid 600 in the first reservoir 610 and thesurrounding environment. For example, if the storage reservoir 610 isconfigured as an underground chamber in which the thermal storage liquid600 is in contact with the chamber walls (i.e. surrounding rock), heatmay be transferred from the thermal storage liquid 600 to thesurrounding ground/rock. Providing thermal insulation may help reducethe amount of heat that escapes from the thermal storage liquid 600while it is being stored. This may help prevent thermal stresses fromdeveloping in the rock and thereby help to improve the cavern stability.Similarly, this may also help improve the overall efficiency of thethermal storage subsystem 120 and/or system 10. Preferably, the thermalstorage subsystem 120 may include at least one thermal insulation layer,that may include one or more layers of physical insulating material(such as fiberglass, plastic, refractory material, ceramic and the like)and/or one or more gas layers and/or one or more vacuum layers betweenthe high temperature thermal storage liquid in the storage reservoir andthe ambient environment.

To help provide such thermal insulation, the chamber walls (e.g. bottom620 and sidewall 621) in the embodiments described may be provided witha layer of insulating material. Alternatively, or in addition to suchinsulation, embodiments that utilize a separate inner chamber, such asthe tank 684 in the embodiment of FIGS. 20A and 20B may be configured toinclude gaps 696 due to offset distances 692 and 694 in which air, orany other suitable gas, may collect. Such air gaps may function asbottom and sidewall insulting gas layers, as direct contact, and theassociated conductive heat transfer, between the tank walls 686 and 688and the chamber walls 620 and 621 is substantially eliminated. Suchembodiments may also utilize one or more layers of physical insulatingmaterial on the various walls of the inner chamber 684.

Optionally, the thermal storage subsystem 120 may include a reservoircooling system that can be selectably operated to reduce the temperatureof the storage reservoir 610. The reservoir cooling system may be atleast partially automatically controlled by the controller 118 (oranalogous controller) based on characteristics of the thermal storagesubsystem 120, such as temperatures and/or pressures within the storagereservoir that are above a pre-determined upper threshold.

The reservoir cooling system may include any type of closed loop coolingsystem, including heat exchangers and the like. It may also be operableto introduce relatively cold liquid into the storage reservoir 610 todirectly mix with the thermal storage liquid 600 or onto the outerchamber or inner chamber walls to provide surface cooling, and/or may beoperable to drain at least some of the hot thermal storage liquid 600from the storage reservoir 610 into a secondary cooling/mixing chamber.Providing a direct mixing and/or draining of liquid from within thestorage reservoir 610 may provide relatively fast cooling, and may bewell suited for cooling in emergency overheating/over pressurizationconditions. Optionally, the reservoir cooling system for the thermalstorage subsystem 120 may include a quantity of cooling liquid that isstored at a cooling temperature (that is lower than the storagetemperature and may be similar to or the same as the source temperature)in a cooling chamber. The cooling liquid may be the same as the thermalstorage liquid 600, or may be a different liquid. The cooling chambermay be the same as the storage reservoir, or the out chamber, or may bethe same as the source reservoir, or it may be a different chamber.Optionally, the reservoir cooling system for the thermal storagesubsystem 120 may include a gas circulation system which conveys thecover gas 602 to a heat exchanger which exhausts a portion of thethermal energy contained with the cover gas to the environment, such asan aerial cooler.

Referring to FIG. 21 , embodiments of a reservoir cooling system areconfigured such that the source reservoir 606 functions as a coolingchamber 674, and contains extra thermal storage liquid (not yet heatedby the heat exchanger 635) that functions as the cooling liquid. A pump676 is provided along a cooling liquid conduit 678 (which may alsoinclude a valve 680 or other equipment) is provided to introduce atleast some of the cooling liquid from the cooling chamber 674 into thestorage reservoir 610, thereby diluting and reducing the temperature ofthe thermal storage liquid 600 in the storage reservoir 610.Alternatively, an automatically opening, pressure-actuated drain valvethat is configured to open at a set condition (possibly pressure) couldbe provided instead of, or in addition to, the pump 676. If the pressurewithin the storage reservoir 610 exceeded a pre-determinedautomatic-cooling pressure threshold, the drain valve may automaticallyopen and allow the heated thermal storage liquid to rush out of thestorage reservoir 610, and preferably to mix with the cooling liquid. Inthe example of FIG. 22 , the source chamber 606 functions as a coolingchamber 674, and the water layer 16 that functions as the coolingliquid. In this example, a cooling liquid conduit 678 is provided as aconduit that passes through the isolating barrier 625 that can be openedto allow mixing between the heated thermal storage liquid 600 in thestorage reservoir 610 and the source chamber 606, which is at asubstantially lower temperature. This flow can be one-way, or two-way.In the embodiment of FIG. 19 a cooling liquid conduit 678 is provided asa conduit that passes through the isolating barrier 625 that can beopened to allow mixing between the heated thermal storage liquid 600 inthe storage reservoir 610 and layer of water 16 in the accumulator,which is at a substantially lower temperature

Referring to FIGS. 20A and 20B, in another embodiment the reservoircooling system for the thermal storage subsystem 120 includes a drainapparatus 682 that is in communication with the storage reservoir 610and, in this example, is provided as a drain in the side wall 686 of thetank 684, and can be selectably opened to drain at least some of thethermal storage liquid 600 from the first storage reservoir 610. Thedrained thermal storage liquid 600 may be directed to any suitablesink/drain, and in the embodiment of FIG. 20A is directed into a coolingchamber that is provided by the source reservoir 605 and contains aquantity of a cooling liquid stored at a cooling temperature that isbelow the storage temperature, which in this example is unheated thermalstorage fluid at the source temperature. Preferably, the cooling chambercan be located at a lower elevation than the storage reservoir 610, suchthat the thermal storage liquid 600 can flow from the storage reservoir610 into the cooling chamber under the influence of gravity, andoptionally without the need for a pump or other conveying mechanism.This may help facilitate operation of the reservoir cooling system, andmay enable the thermal storage liquid 600 to be drained even ifelectrical power is not available.

FIG. 22 illustrates an alternative embodiment of a thermal storagesubsystem 120 in which both the storage reservoir 610 and the sourcereservoir 606 are adjacent and to the accumulator 12. The heat exchanger635 is spaced from the accumulator 12, and may preferably be providedabove ground.

FIG. 23 illustrates an alternative embodiment of a thermal storagesubsystem 120 in which both the storage reservoir 610 and the sourcereservoir 606 are spaced apart from each other and from the accumulator12, and are both positioned below ground. In this arrangement, thestorage reservoir 610 is adjacent the shaft 18 and is above theaccumulator 12. A gas passage conduit 626 in this example extends fromthe accumulator 12 to the storage reservoir 610 to provide fluidcommunication between the gas layer 14 and the cover gas layer 602.

FIG. 7 is a schematic representation of a compressed gas energy storagesystem 10B, according to an alternative embodiment. Compressed gasenergy storage system 10B is similar to the other compressed gas energystorage systems described herein, but is configured so that the upperportion 22A of the gas conduit 22 that conveys compressed gas betweenthe thermal storage subsystem 120 and the compressor/expander subsystem100 extends through the ground 200, and not through shaft 18 and water20. Additional variations are possible.

Furthermore, while in embodiments illustrated the thermal storagesubsystem 120 receives compressed gas from, or provides compressed gasto, the compressor/expander subsystem 100, alternatives are possible inwhich thermal storage is more tightly integrated with multiple stages ofcompressor 112 and multiple stages of expander 116 so as to storethermal energy between stages. This may be done to enable the pieces ofequipment at downstream stages of compressor 112 and expander 116 toreceive and handle compressed gas at a temperature that is within theirmost efficient operating ranges. This may help facilitate heat transferand/or storage at two or more stages in the process, which may helpimprove system efficiency.

Referring to FIG. 8 , optionally, an insulating “jacket” 125 (shown indotted lines to not occlude portions of the thermal storage subsystem120) can be wrapped around an portion of thermal storage subsystem 120to provide some of thermal insulation between the liquid 20 in shaft 18and the thermal storage subsystem 120 thereby to promote rapid heatstratification, which may help increase the performance of a PCM heatstorage system. As described above, air A from the ambient enteringcompressor/expander subsystem 100 can be conditioned to become air A′prior to its entry to the compressor 112 by passing the air throughthermal storage subsystem 120 thereby to cause the air A′ to be at atemperature suited for an efficient operating range of a particularstage of compressor 112.

Optionally, the controller 118 may also be configured to change thecondition of the thermal storage subsystem 120 so as to change thenature of the heat being exchanged between air coming through thethermal storage subsystem 120 into the compressor 112 and the thermalstorage material in the thermal storage subsystem 120, or to changerouting of air to the compressor 112 so that it is not passing throughthermal storage subsystem 120. FIG. 9 is a schematic view of componentsof an alternative compressor/expander subsystem 100 for a compressed gasenergy storage system 10, with multiple compression stages and each isassociated with a respective heat exchanger of a thermal storagesubsystem 120. In particular, when operating in charging mode, incomingair from the ambient A is conveyed first, optionally via a heatexchanger to modify the temperature of the incoming air, into compressor112 a driven by motor 110 a for a first stage of compression. In thisexample, the thermal storage subsystem 120 may include two or more heatexchangers 635 that can be provided between the different compressionstages. Following the first stage of compression, air A is then conveyedthrough a first heat exchanger 635 a of a thermal storage subsystem 120to transfer heat from the air A into the thermal storage liquid 600,thereby to be conditioned to be air A′ which is then conveyed intocompressor 112 b driven by motor 110 b for a second stage ofcompression. Following the second stage of compression, air A′ is thenconveyed through any additional heat exchangers of the thermal storagesubsystem 120 such as second heat exchanger 635 b of thermal storagesubsystem 120 to transfer heat from the air A″ into the thermal storageliquid 600. A last heat exchanger of the thermal storage subsystem 120is represented in this example as heat exchanger 635 x transfer heatsfrom the air A′″ into the thermal storage liquid 600. Following thisx^(th) stage of compression and thermal storage, the air A′″ is conveyeddown into accumulator 12 as has been described above with respect toother embodiments. Optionally, the heat stored in the thermal storagesubsystem 120 in the charging mode may be stored entirely forre-incorporating into air being released when the compressed gas energystorage is operated in a discharging mode, but may in some capacity orquantity be employed for some other purposes of the compressed gasenergy storage system such as for helping to regulate temperature ofanother subsystem, or to operate pneumatic tools and instruments,amongst other uses. It should be noted that, while three stages ofcompression with respective thermal storage stages are shown in FIG. 6 ,a compressed gas energy storage system according to this embodiment ofthe invention may have only two, or more than three stages ofcompression with respective thermal storage stages. Furthermore, inalternative embodiments a given stage of compression is not necessarilyalways followed by a stage of thermal storage. Furthermore, inalternative embodiments, incoming air that has not yet been compressedin the compressed gas energy storage system may first pass through athermal storage subsystem or stage thereof to reduce or increase itsheat content prior to entering a compressor, rather than a heatexchanger that might dissipate the heat from the system.

FIG. 10 is a schematic view of components of an alternativecompressor/expander subsystem for a compressed gas energy storagesystem, with multiple expansion stages each associated with a respectiveheat exchanger of a thermal storage subsystem 120. In particular, duringan expansion (release) phase, compressed air A released from accumulator12 is first conveyed through a first exchanger 635 a of a thermalstorage subsystem 120 to transfer heat from the thermal storage liquid600 into the air being conveyed thereby to be conditioned as air A′. AirA′ is presented to a first expander 116 a driving a generator 114 a fora first stage of expansion. Following the first stage of expansion, airA′ is then conveyed through a second exchanger 635 b to transfer storedheat from the thermal storage liquid 600 into the air being conveyedthereby to be conditioned to be air A″, which is then conveyed intoexpander 116 b driving generator 114 b for a second stage of expansion.Following the second stage of expansion, air A″ is then conveyed throughany additional stages of the thermal storage subsystem 120. A lastexchanger of the thermal storage subsystem 120 is represented in thisexample as exchanger 635 x which transfers stored heat into compressedair being conveyed through expansion stage 635 x thereby to beconditioned to be air A′″. Following this x^(th) stage of expansion andheat release from thermal storage, the air A′″ is conveyed to theambient atmosphere A as has been described above with respect to otherembodiments. The heat stored in the thermal storage subsystem 120 mayhave been stored from incoming air being compressed during a storagephase of the compressed gas energy storage system, but alternatively orin some combination may have been stored during operation of anotheraspect or subsystem of the compressed gas energy storage system, such asduring temperature regulation of another subsystem, or during anelectrical heating process. It should be noted that, while three stagesof expansion with respective thermal storage stages are shown in FIG. 10, a compressed gas energy storage system according to this embodiment ofthe invention may have only two, or more than three stages of expansionwith respective thermal storage stages. Furthermore, in alternativeembodiments a given stage of expansion is not necessarily alwayspreceded in the processing chain by a stage of release of heat fromthermal storage.

FIG. 11 is a schematic view of components of an alternativecompressor/expander subsystem for a compressed gas energy storagesystem, with pairs of compression and expansion stages each associatedwith a respective exchanger of the thermal storage subsystem 120. Inthis embodiment, a given exchanger of the thermal storage subsystem 120is used during both the compression and expansion stages, by routing airbeing conveyed into the accumulator 12 through the thermal storagesubsystem 120 to remove heat from the air either prior to a subsequentstage of compression or prior to storage, and routing air being conveyedout of accumulator 12 through the thermal storage subsystem 120 to addheat to the air either after release from accumulator or after a stageof expansion. In a sense, therefore, pairs of compression and expansionstages share a heat exchanger 635 a, 635 b and 635 x and airflow iscontrolled using valves V, as shown in the Figure. This embodiment maybe useful where the “same” heat stored from compressed air beingconveyed towards the accumulator 12 during a storage phase is to bereleased into the air being released from the accumulator 12 during arelease phase.

FIG. 12 is a schematic view of components of the alternativecompressor/expander subsystem of FIG. 11 , showing airflow during anexpansion (release) phase from storage through multiple expander stagesand multiple respective heat exchangers of the thermal storage subsystem120. In this phase, through control of valves V, airflow is directedthrough multiple expansion stages in a manner similar to that shown inFIG. 10 . The dashed lines show multiple compression stages the airflowto which is prevented during an expansion phase by the control of valvesV.

FIG. 13 is a schematic view of components of the alternativecompressor/expander subsystem of FIG. 11 , showing airflow during acompression (storage) phase from the ambient A through multiplecompressor stages and multiple respective heat exchangers of the thermalstorage subsystem 120. In this phase, through control of valves V,airflow is directed through multiple compression stages in a mannersimilar to that shown in FIGS. 1 and 12 . The dashed lines show multipleexpansion stages the airflow to which is prevented during thecompression phase by the control of valves V.

FIG. 14 is a sectional view of components of an alternative compressedgas energy storage system 10C, according to an embodiment. In thisembodiment, compressed gas energy storage system 10C is similar to theother embodiments of the compressed gas energy storage systems describedherein. However, in this embodiment the thermal storage subsystem 120(including any of the suitable variations described herein, including astorage reservoir 610, source reservoir 606 and related equipment) islocated within the accumulator 12 and may be at least partially immersedwithin the compressed gas in compressed gas layer 14. The thermalstorage subsystem 120 may be positioned within the accumulator 12 duringconstruction via the opening 27 that is thereafter blocked with bulkhead24 prior to filling shaft 18 with liquid 20. The thermal storagesubsystem 120 can thus be designed to allow for the construction,insulation, etc. to be completed prior to placement within theaccumulator 12 and/or is constructed in easily-assembled componentswithin the accumulator 12. This allows for the units to be highlyinsulated and quality-controlled in their construction, which enablesthe thermal storage subsystem 120 to be generally independent of theaccumulator 12, with the exception of an anchoring support (not shown).

Optionally, a regulating valve 130 associated with the interior ofthermal storage subsystem 120 may be provided and configured to openshould the pressure within the thermal storage subsystem 120 becomegreater than the designed pressure-differential between its interior andthe pressure of the compressed gas layer 14 in the surroundingaccumulator 12. Pressure within the thermal storage subsystem 120 may bemaintained at a particular level for preferred operation of the latentor sensible material. For example, heated water as a sensible materialmay be maintained at a particular pressure to maintain the thermal fluidin its liquid state at the storage temperature. The regulating valve 130may open to allow the pressurized gas in the interior to escape to theaccumulator 12 and can close once the pressure differential is loweredenough to reach a designated level. In an alternative embodiment, such aregulating valve may provide fluid communication between the interior ofthe thermal storage subsystem 120 and the ambient A at the surfacethereby to allow gas to escape to the ambient rather than into theaccumulator 12. While thermal storage subsystem 120 is shown entirelyimmersed in the compressed gas layer 14, alternative thermal storagesubsystems 120 may be configured to be immersed partly or entirelywithin liquid layer 16. In some examples, only a portion of the thermalstorage subsystem 120, such as the storage reservoir 610, may be atleast partially nested within the accumulator 12, and other portions,such as the heat exchangers and the source reservoir 606, may be spacedapart from the accumulator 12.

FIG. 15 is a sectional view of components of an alternative compressedgas energy storage system 10D, according to another alternativeembodiment. In this embodiment, the compressed energy gas storage system10D includes a different type of accumulator 12D that is nothydrostatically compensated, and may be a salt cavern, an existinggeological formation, or manmade. That is, the accumulator 12D isconfigured to contain compressed gas but need not include a liquid layeror be associated with a shaft containing water. This is another type ofaccumulator that may, in some embodiments, be used in place of theaccumulators 12 used with respect to other embodiments of the compressedgas energy storage systems described herein. Aspects of the thermalstorage subsystems 120 described in this embodiment may be used incombination with the hydrostatically compensated compressed gas energystorage systems described, and aspects of the thermal storage subsystems120 depicted in other embodiments may be utilized with accumulatorssimilar to accumulator 12D. In this embodiment, compressed gas energystorage system 10D is similar to above-described compressed gas energystorage systems. However, the thermal storage subsystem 120 is locatedwithin an isobaric pressurized chamber 140 within ground 200 that may bemaintained at the same pressure as is accumulator 12, or a pressure thatis substantially similar to the accumulator pressure or optionally at apressure that is less than or greater than the accumulator pressure.Optionally, the thermal storage subsystem 120 may be positioned withinthe pressurized chamber 140 during construction via an opening that isthereafter blocked so the chamber 140 may be pressurized to a workingpressure that is, preferably, greater than atmospheric pressure. Thethermal storage subsystem 120 can thus be designed to allow for theconstruction, insulation, etc. to be completed prior to placement withinthe chamber 140 and/or is constructed in easily-assembled componentswithin the chamber 140. This allows for the units to be highly insulatedand quality-controlled in their construction, which enables the thermalstorage subsystem 120 to be generally independent of the chamber 140,with the exception of anchoring support (not shown). A regulating valve130 associated with the interior of thermal storage subsystem 120 isprovided and configured to open should the pressure within the thermalstorage subsystem 120 become greater than the designedpressure-differential between the interior and the surroundingpressurized chamber 140. Pressure within the thermal storage subsystem120 may be required to be maintained at a particular level for optimaloperation of the latent or sensible material. For example, heated wateras a sensible material may be required to be maintained at a particularpressure to maintain the thermal fluid in its liquid state at thestorage temperature. The regulating valve 130 opens to allow thepressurized gas in the interior to escape to the pressurized chamber 140and closes once the pressure differential is lowered enough to reach adesignated level. In an alternative embodiment, such a regulating valve130 may provide fluid communication between the interior of the thermalstorage subsystem 120 and the ambient A at the surface thereby to allowgas to escape to the ambient rather than into the pressurized chamber140.

Locating the thermal storage subsystem 120 above the accumulator 12, andthus physically closer to the compression/expansion subsystem 100, mayhelp reduce the length of piping required, which may help reduce thecosts of piping, installation and maintenance, as well as reducedfluid-transfer power requirements.

While the embodiment of compressed gas energy storage system 10Dincludes an isobaric pressurized chamber 140, alternatives are possiblein which the chamber 140 is not strictly isobaric. Furthermore, inalternative embodiments the pressurized chamber 140 may be in fluidcommunication with gas layer 14 and thus can serve as a storage area forcompressed gas being compressed by compressor/expander subsystem 100along with accumulator 12. In this way, the pressure of the gas in whichthe thermal storage subsystem 120 is immersed can be maintained throughthe same expansions and compressions of gas being conveyed to and fromthe accumulator 12.

Optionally, any of the thermal storage subsystems 120 described hereinmay include a thermal conditioning system that can be used to regulatethe temperature of the layer of cover gas 602 in the storage reservoir610 and/or in the source reservoir 606. For example, the thermalconditioning system may include a fan cooler, heat exchanger, evaporatorcoils or other such equipment so that it can be used to optionallyreduce (or alternatively increase) the temperature of the layer of covergas 602 when the thermal storage subsystem 120 is in use.

We claim:
 1. A compressed gas energy storage system comprising: a) anaccumulator having an interior configured to contain compressed gas whenin use, wherein the accumulator has a primary opening, an upper wall, alower wall and the accumulator interior containing a compressed gasbeing at least partially bounded by the upper wall and lower wall; b) agas compressor/expander subsystem spaced apart from the accumulator andcomprising at least a first compression stage having a gas inlet and agas outlet in fluid communication with the accumulator interior via agas conduit for conveying compressed gas to the accumulator when in acharging mode and from the accumulator when in a discharging mode; andc) a thermal storage subsystem comprising: i. at least a first storagereservoir configured to contain a thermal storage liquid at a storagepressure that is greater than atmospheric pressure, wherein the firststorage reservoir comprises a pressurized layer of cover gas above thethermal storage liquid and the layer of cover gas is formed by a boilingof a portion of the thermal storage liquid within the first storagereservoir whereby the layer of cover gas is pressurized to the storagepressure; ii. a liquid passage having an inlet connectable to a thermalstorage liquid source and configured to convey the thermal storageliquid to the first storage reservoir; and iii. a first heat exchangerprovided in the liquid passage and in fluid communication between thefirst compression stage and the accumulator, whereby when the compressedgas energy storage system is in a charging mode thermal energy istransferred from a compressed gas stream exiting the gascompressor/expander subsystem to the thermal storage liquid.
 2. Thecompressed gas energy storage system of claim 1, wherein the thermalstorage liquid is heated to a storage temperature prior to entering thefirst storage reservoir, wherein the storage temperature is below aboiling temperature of the thermal storage liquid when at the storagepressure and is above the boiling temperature of the thermal storageliquid when at atmospheric pressure.
 3. The compressed gas energystorage system of claim 2, wherein the storage temperature is betweenabout 150 degrees Celsius and about 350 degrees Celsius.
 4. Thecompressed gas energy storage system of claim 1, wherein the compressedgas within the accumulator is at an accumulator pressure, and whereinthe storage pressure is less than the accumulator pressure.
 5. Thecompressed gas energy storage system of claim 1, wherein the firststorage reservoir is at least partially disposed within the accumulator.6. The compressed gas energy storage system of claim 1, wherein thethermal storage liquid source comprises a source reservoir containing aquantity of the thermal storage liquid at a source temperature that isless than a storage temperature.
 7. The compressed gas energy storagesystem of claim 6, wherein the source reservoir is external the firststorage reservoir.
 8. The compressed gas energy storage system of claim1, wherein the gas compressor/expander subsystem comprises a secondcompression stage downstream from the first compression stage and thefirst heat exchanger is fluid communication between the firstcompression stage and the second compression stage, and the thermalstorage subsystem further comprises a second heat exchanger in fluidcommunication between the second compression stage and the accumulator,whereby thermal energy is transferred between the compressed gas streamexiting the second compression stage and the thermal storage liquid. 9.The compressed gas energy storage system of claim 8, wherein the gascompressor/expander subsystem comprises a third compression stagedownstream from the second compression stage and the second heatexchanger is fluid communication between the second compression stageand the third compression stage, and the thermal storage subsystemfurther comprises a third heat exchanger in fluid communication betweenthe third compression stage and the accumulator, whereby the thermalenergy is transferred between the compressed gas stream exiting thethird compression stage and the thermal storage liquid.
 10. Thecompressed gas energy storage system of claim 1, wherein the firststorage reservoir comprises a single chamber having a chamber bottomwall, a chamber top wall, a chamber sidewall extending therefrom anddefining a chamber interior configure to contain the thermal storageliquid.
 11. The compressed gas energy storage system of claim 10,wherein the single chamber includes a natural underground cavity formedat least partially of natural rock.
 12. The compressed gas energystorage system of claim 11, further comprising a storage liner coveringat least a portion of an interior surface of the chamber.
 13. Thecompressed gas energy storage system of claim 1, further comprising anextraction pump in liquid communication with the thermal storage liquidin the first storage reservoir and selectably operable to pump thethermal storage liquid at a storage temperature out of the first storagereservoir, and wherein when an exit stream of gas is released from theaccumulator, thermal energy is transferred from the thermal storageliquid pumped out of the first storage reservoir into the exit stream ofgas.
 14. The compressed gas energy storage system of claim 13, whereinthe exit stream of gas and the thermal storage liquid pumped out of thefirst storage reservoir pass through the first heat exchanger.
 15. Thecompressed gas energy storage system of claim 1, wherein the firststorage reservoir is disposed entirely under ground.
 16. The compressedgas energy storage system of claim 1, further comprising a reservoircooling system for selectably cooling a temperature of the thermalstorage liquid contained in the first storage reservoir, therebyreducing the storage pressure within the first storage reservoir. 17.The compressed gas energy storage system of claim 1, wherein when thecompressed gas energy storage system is in the discharging mode,compressed gas travels from the accumulator to the gascompressor/expander subsystem and at least a portion of the thermalstorage liquid at a storage temperature is withdrawn from the firststorage reservoir and the thermal storage subsystem is operable so thatthermal energy is transferred from at least the portion of the thermalstorage liquid withdrawn from the first storage reservoir to thecompressed gas exiting the accumulator whereby a temperature of thecompressed gas exiting the accumulator is increased before it reachesthe gas compressor/expander subsystem.
 18. The compressed gas energystorage system of claim 17, wherein when the compressed gas energystorage system is in the discharging mode the compressed gas travelingfrom the accumulator to the gas compressor/expander subsystem passesthrough the first heat exchanger to receive the thermal energy from thethermal storage liquid.
 19. The compressed gas energy storage system ofclaim 1, wherein the first storage reservoir is configured as anabove-ground vessel.